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Ion Exchange and Mechanical Purification of Fire-Resistant Phosphate Ester Fluids used in Steam

Turbine Control Systems

W.D. Phillips, W David Phillips & Associates G.J.W. Staniewski, Ontario Power Generation

S. Suryanarayan, Kinectrics Inc.

Abstract

Steam turbines at nuclear stations have electro-hydraulic control (EHC) systems that use a phosphate

ester-based fire-resistant fluid. This fluid undergoes degradation in service via hydrolytic, oxidative and

thermal mechanisms that are influenced by system design and operating conditions. Past experience

(OPEX) has shown that the condition of the fire-resistant fluid in service is critical for station safety and

nuclear regulatory authorities therefore include chemistry control of this fluid as a part of a station’s

operating licence.

The typical industry approach to maintaining fluid quality within specification is to continuously circulate

a portion of the fluid through purification media. Since the late 1980s ion exchange treatment has

become one of the most effective purification processes. However, there are now several different resin

types available which can interact with the fluid in different ways and the optimum process for resin

treatment of phosphate esters has still to be identified. In fact, it will probably be necessary to have

several different options depending on the operating conditions.

However, EHC fluid purification is not limited to acidity control. It is also important to keep the fluid

clean and dry if it is to operate efficiently and offer a long service life. Mechanical techniques are

therefore needed to complement and maintain the activity of the resin treatment. For example, resin

fouling by particulate can reduce its activity and this may require improved filtration.

The main objective of this paper is to present the initial results of a new comparison of resin behaviour

intended to improve performance of the ion exchange treatment at CANDU nuclear stations. Also

included are the results of early investigations into different techniques for drying the fluid and for

removing small particles arising from fluid degradation. The paper will additionally provide a brief

description of the design requirements of the steam turbine electro-hydraulic control system together

with an explanation of the degradation mechanisms of phosphate esters, the products of degradation

and their impact on fluid life and performance. An introduction to the principal factors affecting the

efficiency of different ion exchange treatments follows, and the paper concludes with a discussion on

the work required before a final resin selection can be made.

1 Introduction

In the 1950s an increasing demand for greater stability of the power supply forced the steam turbine

manufacturers to provide a more efficient turbine control system. Historically, the function of

controlling turbine speed and the rate of load increase was performed by a mechanical hydraulic control

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(MHC) system. It utilized a complex arrangement of mechanical devices operating at relatively low

pressures. Unfortunately this system had a limited control capability.

In order to provide a faster response to a changing load on the electric grid and to provide more precise

control, an advanced turbine control system was developed for the majority of new turbines in nuclear

power generating stations. This new system, called electro-hydraulic control (EHC), utilized a complex

electronic control system and a dedicated high pressure (typically between 1000 to 2000 psi) hydraulic

system independent of the turbine lubrication system.

During the initial development stage of EHC systems more attention was given by turbine manufacturers

to the design of the electronics, considering the hydraulic portion as a relatively mature design.

Consequently, different designs of EHC systems exist today that have a different impact on system

reliability.

Regardless of this new approach, turbine trips occasionally occur resulting in a total loss of production.

According to the Electric Power Research Institute (EPRI) a significant contributor to such trips are EHC

system failure caused by both electronic and hydraulic component issues [1]. The objective of this paper

is to address some of the critical hydraulic issues affecting the EHC system reliability.

2 Characterization of Typical Electro-Hydraulic Systems

The EHC system is a complex design consisting of two major systems: electric/electronic control systems

and the hydraulic fluid system. In addition, it also has an emergency trip system, valve controllers and a

monitoring system. Although this system is vital for power generation it is not considered a safety-

related system. However, many nuclear regulatory agencies include requirements for maintaining the

hydraulic fluid within the industry specification in the station operating licence.

The hydraulic portion of a typical EHC system consists of the steam control valve system, emergency trip

system valves, pumps, accumulators, coolers, filters, instrumentation, piping, reservoir and the fluid

purification system. Each of these components has a different impact on the fluid deterioration rate.

Usually pumps, reservoirs, pressure relief valves and the piping arrangement create the largest impact

on fluid degradation rate while steam control valves, specifically the servo valves and solenoid valves are

the most sensitive components, failures of which are usually cited by EPRI as the largest category

problem in the EHC systems [1,2,3]. Servo valves are particularly sensitive to the fluid condition and

therefore monitoring the fluid properties is essential to prevent this failure mode [4].

In addition, the EHC system has a few interconnections with other plant systems such as the cooling

water system for the heat exchangers, compressed air systems for the air operated valves and the low

voltage electrical distribution system for powering various motors and solenoid valves. Malfunction of

these systems also influences the reliability of the EHC system.

2.1 Hydraulic Fluid Pumps

The pump trains of all EHC systems are designed with either two pumps, each capable of 100% capacity,

or three pumps, each capable of 50% capacity. Normally, different types of positive displacement

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pumps (e.g., vane, piston, screw) are used in the majority of EHC systems. Usually EHC systems have a

parallel fluid delivery system associated with independent pumps, filters, pressure relief valves and

instrumentation. Normally only one pump line is in operation with the other in standby mode. The

standby pump starts when lower pressure is detected in the common discharge header.

Until recently many turbine manufacturers used constant volume pumps. In such systems typically 95%

of the fluid is returned to the reservoir over a pressure relief valve. Apart from their high energy

consumption, such systems create additional stress on the fluid which is manifested by a higher fluid

degradation rate. The continuously high flow rates encourage the entrainment of air as a result of

turbulence and with poor tank design this can lead to a condition known as micro-dieseling where

compression of the air bubbles in the pump creates very high temperatures and localised degradation.

Although adiabatic compression, the most extreme condition, is unlikely to be seen the temperatures

may still be high enough for the complete destruction of the fluid molecule [5].

General Electric was the first turbine manufacturer to introduce variable volume pressure compensated

pumps into their EHC systems. These pumps vary the discharge flow capacity based on system demand.

Such a system not only reduces system power consumption but also provides a more stable discharge

pressure and significantly less stress to the fluid. Currently, this type of pumping arrangement is the

preferable design approach for all new and retrofitted EHC systems.

Reportedly, the major source of pump problems is related to contamination of hydraulic fluids [2]. Other

common problems are related to pump cavitation, particularly due to negative suction, air entrainment

and cold fluids. Excessive vibration is also cited as another common pump problem [6].

2.2 Hydraulic Fluid Reservoir and Piping

A typical EHC fluid reservoir provides a storage, supply and return location for the EHC hydraulic fluid. It

is a stainless steel tank with internal baffling to increase fluid residence time and promote release of

entrained air. The size of the fluid reservoir varies significantly between different turbine manufacturers

and depends on selected fluid/pressure flow design criteria. The reservoir size ranges from

approximately 750 liters (for small Westinghouse EHC systems) to 75,000 liters (for large Siemens EHC

systems). Each reservoir would have a number of cover plates (to provide access for cleaning and

inspection).

A small reservoir may promote adiabatic compression particularly if the fluid residence time is short and

is insufficient to release any air bubbles. In most cases, the use of such reservoirs would relatively

quickly result in signs of fluid degradation. However, there are some advantages. The fluid may respond

faster to purification treatment due to the smaller volume and the cost of purification media would be

significantly lower. Also from a maintenance point of view, transferring a smaller amount of fluid to a

temporary storage tank is a relatively easy process.

The larger reservoirs can improve fluid residence time allowing a more effective separation of entrained

air and precipitation of solid particles. However, their use may delay the appearance of significant fluid

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deterioration. In addition, any fluid purification process would be more difficult to perform, time

consuming and more expensive.

Regardless of reservoir size, fluid level should be maintained close to the upper limit to maximise its

residence time. Due to the inherently hygroscopic nature of phosphate esters, moisture control in the

fluid reservoir is an important factor. In many cases, air entering the tank passes through a desiccant

breather, which is usually associated with a 5 micron air filter (to reduce contamination by dust). Fan-

assisted extraction may also be used to maintain a constant flow of air across the fluid surface. Yet

other designs pipe dry air into the reservoir. Occasionally, reservoirs may also be purged with ambient

air. Such a system can be effective if the surrounding air has a relatively low humidity, otherwise the

fluid may be significantly contaminated.

Recently, active head-space membrane dryers were introduced as a cost-effective source of dry air. This

system applies membrane technology using a hollow, fiber-based, membrane dryer to separate

moisture from the compressed air. In addition, this system also applies a pre-filter for the removal of

solid particles and any oily residues from the air before allowing the dry air to purge the fluid reservoir.

Some reservoirs may have electric heaters. They may burn the fluid layer adjacent to the heater tubes if

the heat density is too high. Usually low-density heaters are applied for this application or the heat is

applied indirectly. Some systems heat the fluid by re-circulating it within a closed circuit. This would

require additional small centrifugal pumps forcing the fluid to flow through the reservoir.

Another problem may be related to the existence of large gaps, including the drain openings, in the

reservoir baffle plates. These can cause the fluid to by-pass the designed fluid path and shorten the fluid

residence time.

The piping and tubing in the EHC systems are normally made from stainless steel. The location and the

depth of the return line in the tank are important factors affecting fluid degradation. Locating the

return line close to the pump inlet could significantly reduce the fluid residence time and promote

aeration, particularly if it is not submerged at all times in the fluid. Another factor is the slope of the

return line to the reservoir. A greater slope will promote more turbulence and aeration. This may create

an additional problem at the reservoir with the air bubble separation process. Close proximity to a

steam pipe, particularly if it is not properly insulated, would cause local heating of the fluid [7].

There are several reports describing problems relating to the use of unsuitable sealing materials or

inadequate maintenance. These can also adversely affect the fluid condition and reduce the EHC system

reliability [8].

2.3 Steam Control Valves

Steam valve configuration and operation vary widely depending on turbine manufacturer and model.

The main stop- and reheat valves are used to shut the steam flow to the high- and low-pressure turbines

respectively. The function of controlling the steam flow rate to high pressure turbines is provided by the

governor valve while a similar function for low pressure turbines is done by the intercept valves. The

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exact operating sequence and function of these valves during different stages of turbine operation may

vary and is a complex process outside the scope of this paper.

In addition, turbine manufacturers have different approaches for positioning turbine control valves (e.g.,

the governor and interceptor valves). Some of them applied servo valves with close dimensional

tolerances, others electrical solenoid valves operated by hydraulic servo-motors.

Servo valves receive an electronic signal from the control system causing a movement of the valve

nozzle and position the spool valve. This will set the servo valve in either the open or closed position.

Servo valves also have a bias spring, which are designed to position the spool valve such that the valve

will close on a loss of electrical signal to servo coil. Excessive internal leakage is usually caused by

electrochemical erosion as a result of deteriorated fluid condition [4]. Another frequent problem is a

sticky servo valve condition as a result of the presence of metal soaps or other degradation products,

which overcome the ability of the electric signal or the bias springs to move the spool.

A solenoid valve is a valve having an electrical coil and a plunger. The solenoid portion is an

electromagnet. When a voltage is applied, a magnetic field is generated causing the plunger to re-

position. An attached spring forces the plunger return to its original position when the voltage is

removed. The most frequently reported failure modes are binding or internal leakage and both failures

are attributed to contamination of the hydraulic fluid.

3 Fire-Resistant Fluids for EHC Systems

High fluid pressures applied in EHC systems increase the risk of a fire arising from the escape of a jet or

spray of hydraulic fluid onto a hot steam pipe. To reduce the fire risk turbine manufacturers selected a

fire-resistant hydraulic fluid for this application.

Originally two different fluid types were used: chlorinated aromatic hydrocarbons and triaryl phosphate

esters. The use of the former was discontinued in the 1970s because of servo-valve erosion problems

and environmental concerns [9]. As a result phosphate ester fire-resistant fluids have been used for over

50 years as turbine control fluids at power stations. While they have been successful in preventing fires

their use has not been trouble free.

Triaryl phosphate esters were initially introduced as fire-resistant hydraulic fluids towards the end of

World War II following a series of fires and explosions in military hardware, particularly aircraft, and

their use in industrial applications slowly followed. Their initial use in power generation probably

occurred in Germany as a steam turbine main bearing lubricant (and hydraulic fluid) following major

turbine fires. A successful trial was carried out in 1942 in a 6 MW industrial steam turbine and a report

made to the German Power Station Association in 1943 of over 6000 operating hours without problems

[10].

Phosphate esters are also known as esters of phosphoric acid and can be considered as having the

following general structure (Figure 1).

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O

II

R1O – P – OR3

I

OR2

FIG. 1 The Generic Structure of Neutral Phosphate Esters

Although trialkyl phosphates are available, only triaryl phosphates are used in power generation. In

these fluids, R1 – R3 are phenyl or substituted phenyl (e.g., methylphenyl, also known as cresyl or tolyl),

dimethylphenyl (better known as xylyl), isopropylphenyl, or tertiarylbutylphenyl groups.

The raw materials (phenol and substituted phenols) available for the manufacture of triaryl phosphates

have changed over the years and, as a result, different types of triaryl phosphate have become

commercially available. Most of these phosphates have at some time been used in EHC systems but in

the last twenty years industry has focused on three main types, tri-isopropylphenyl phosphate,

tritertiarybutylphenyl phosphate and trixylenyl phosphate. Although all provide a high level of fire

resistance, their physical and chemical properties do vary and, depending on operating conditions, this

can influence their selection. These products have therefore been used either as 100% fluids or in blends

with each other.

4 Phosphate Ester Degradation Processes and the Products of Degradation

Phosphate esters can degrade in several ways. While this discussion used to be limited to a simple study

of hydrolysis (the reaction with water) and oxidation under laboratory test conditions, there is now an

awareness that conditions in service can be considerably more severe than those simulated in the

laboratory. For example, the very high temperatures found as a result of micro-dieseling (rapid

compression of air bubbles) and static discharge considerably increase the complexity of reactions

taking place in the fluid and therefore the range of degradation products that can arise during

decomposition. This section looks at the different degradation mechanisms; the products of

degradation and the sensitivity of the different types of phosphate to the modes of degradation. More

detailed information on the effect of degradation on phosphates is available in [11].

4.1 Hydrolytic Stability

In the presence of small amounts of water, phosphate esters tend to hydrolyze [i.e., break down into

their constituent acids and alcohols (or, in the case of aryl phosphates, phenols)]. It is the most common

form of degradation of phosphates and the one which most frequently dictates their service life.

The hydrolysis reaction is not rapid at ambient temperatures but accelerates with increasing

temperature and is catalyzed by the presence of strong acids and some metals. As the products of

hydrolysis are themselves strong acids, the process is said to be ‘autocatalytic’. Strong bases can also

catalyze the reaction. The rate of change of acidity increases with time as shown in Figure 2 [11] which

compares the stability of different types of fluid in the standard hydrolytic stability test (ASTM D 2619).

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This is, however, an accelerated test due to the high water content and high surface area of the copper

catalyst to the required volume of liquid.

FIG. 2 The Hydrolytic Stability of Different Phosphate Esters Showing the Change in Rate with Time

The hydrolysis of triaryl phosphates (e.g., triphenyl phosphate) takes place as indicated in Figure 3 [12].

The replacement of successive aryl groups with -OH groups becomes increasingly difficult and the last

step is not normally achieved without using very severe conditions, such as boiling with dilute acid or at

very high temperatures (e.g. under micro-dieseling conditions). As a result, under normal operating

conditions, it is extremely unlikely that phosphoric acid would be generated.

The reaction scheme shown in Figure 3 shows the strong acids (pH < 7)—also known collectively as

partial phosphates—are produced. Although these are of greatest importance in view of their reactivity,

weak acids (pH > 7) such as phenols, are also formed in this process. While they are not thought to have

a major effect on the performance of the fluid, they can adversely affect foaming and volume resistivity,

the latter being used to assess the potential for servo-valve erosion. They are also intermediates in the

formation of polyphosphates arising from the condensation of two or more mono- or dihydrogen

phosphates and in the presence of metal soaps can form metal phosphates.

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FIG. 3 The Hydrolysis of Triphenyl Phosphate

The presence of strong acids can have different effects on fluid and system performance. Some of these

are beneficial, whereas others can cause operational problems. The acids may, for example, inhibit the

corrosion of ferrous metal surfaces but promote corrosion of nonferrous components; they may

adversely affect the electrical properties of the fluid but assist in reducing wear. In general, the

uncontrolled generation of acidic products is harmful to the life and performance of phosphate esters,

and in certain critical applications such as the power generation industry, it is customary to remove

them as they are produced by circulating the fluid through an adsorbent solid thereby prolonging fluid

life and this will be discussed in greater detail later in this paper. However, strong acids are also

chemically reactive and can form metal soaps or salts, for example with components of the fuller’s earth

or activated alumina adsorption media and these are subsequently dissolved or dispersed in the fluid. As

the soaps eventually precipitate (e.g. in servo-valves) causing sticking and also promote foaming and air

retention, their presence is highly undesirable and is a major reason why industry has moved to the use

of ion exchange resins.

The importance of acidity in the fluid degradation cycle is shown in Figure 4 [12] and underlines the

need to reduce the acid level and find alternatives to the fuller’s earth and alumina adsorbents.

`

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FIG. 4 The Fluid Degradation Cycle

While utilising the most efficient acid adsorbent is important, the use of a phosphate with superior

hydrolytic stability is also beneficial and such materials are currently used in the manufacture of

phosphates for the main bearing lubrication of steam turbines where no in situ conditioning or

purification is provided.

4.2 Oxidation Stability

Good oxidation stability is an essential property for hydraulic fluids that are exposed to ‘high’

temperatures. Oxidative degradation results in the production of a range of acids (both strong and

weak) and from very low to high molecular weight; low molecular weight hydrocarbons and their

oxidates that may plate out as varnish and higher molecular weight hydrocarbons or polyphosphates

present as sludge that can increase fluid viscosity and reduce resistivity while blocking filters and causing

valve sticking. At the extremes of temperature, extremes of oxidants will be formed, that is gaseous

degradation products (e.g. hydrogen) and carbon (see below). However, in most hydraulic systems fluids

are intermittently subjected to temperatures up to 150-200 0C as a result of being compressed in

pumps, bearings, relief valves and restrictors [13], and sometimes from tank heating. In addition, fluid

can be exposed to heating from external sources such as a steam line, hot/molten metal or a welding

torch located close to the hydraulic line. The difference between internal ‘hot spots’ and external heat

sources is that the former occur when the fluid is in circulation (except for the fluid being heated up in

the tank) while an external heat source can also be present when the fluid is static. If the fluid is

circulating, oxidation will normally occur. However, if the fluid is static, oxidation may occur until the

oxygen is consumed, after which pyrolysis (or thermal degradation) may take place.

Oxidation is a free radical process occurring as a result of energy applied in the form of heat. The first

step normally involves an attack on the alkyl substituent on the ring. The isopropyl group is particularly

vulnerable due to the labile secondary hydrogen atom; the methyl group is more stable and the tertiary

butyl group is very resistant to attack. Unsubstituted phenyl groups are also extremely stable and for

both this and the tertiarybutylphenyl phosphate the initial degradation arises through thermal

breakdown (fission) of the P—O bond.

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The isopropylphenyl phosphates and TCP/TXP are responsive to the addition of stabilizers, and in service

can contain inhibitors which improve performance and significantly extend life. In contrast, the

tertiarybutylphenyl phosphates are not as responsive to classical antioxidants but have such good

oxidation stability that this is rarely a disadvantage.

One additional aspect of oxidation stability that is of concern in some applications is the deposit-forming

tendency at high temperatures. This property, also known as “coking,” occurs when a thin film of fluid is

heated on a metal surface while exposed to air. Depending on the stability of the product, the deposit

can vary from soft and carbonaceous to a hard, brittle layer, or to a lacquer. The formation of such

deposits can occur on heater and valve surfaces and can reduce component efficiency. In situ

conditioning, however, has been shown to control the deposit formation [14]. Another form of deposit,

commonly known as ‘varnish’ occurs when fluid degradation products precipitate onto the surface of

metal components and then harden under the influence of heat. It can be difficult to differentiate

between the two forms of deposit without complex analytical investigations. Table 1 [15] shows the

variation in coking tendencies for a range of triaryl phosphates and commercial mineral gas turbine oils.

TABLE 1 Coking Tendency Data for Triaryl Phosphates: Test Method: Federal Test Method Standard VV-L-791C,

Method 3462 [15]

Phosphate ester Deposit formation (mg) at

300C 316C 325C

TXP (uninhibited) 460 1820 1750

TXP (inhibited) 43 1006 1420

TBPP/32 (uninhibited) 3 16 6

TBPP/46 (uninhibited) 3 4 2

IPPP/46 (inhibited) 4 - -

Mineral gas turbine

lubricants

ISO VG 32 25 170 130

ISO VG 46 49 185 227

Mention was earlier made of a phenomenon called micro-dieseling. This involves the compression of air

bubbles in the pump which have not been released during the retention time in the tank. Unless the

bubbles dissolve readily (which depends on their size) it is accompanied by a significant increase in the

temperature of the wall of the bubble. When no heat is lost from the bubble as it is compressed, the

process is known as adiabatic compression and temperatures within the air bubble as high as 10000C

have been calculated for pressures of about 140 bar [16]. However, in practice some heat is lost to the

surrounding fluid and temperatures are unlikely to be so high (unless the pressure is increased still

further). Certainly, evidence exists for temperatures around 800-850 0C [5] but even this temperature

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greatly exceeds the thermal stability of the molecule and it will cause fragmentation with the lower

molecular weight species reacting with oxygen or polymerizing. In mineral oil these high temperatures

have been shown to cause combustion (a discharge) within the air bubble but to date there have not

been any reports of this occurring with phosphates.

In addition to the decomposition products found at lower temperatures, the very high temperatures

(found on, and adjacent to, the bubble wall) of this process also mean that other reactions can take

place involving nitrogen rather than oxygen. For example, fixation of nitrogen has been reported as well

as the formation of nitrogen oxides [16]. These are the ’raw materials’ for a range of additional

degradation products most of which have been identified in the analysis of phosphate samples taken

from systems where dieseling is occurring. Some of these degradation products are indicated in Figure 5

[5] while some of the gaseous degradation products have been identified by dissolved gas analysis and

are listed in Table 2 [5].

FIG. 5 Some Possible High Temperature Degradation Mechanisms

One of the features of dieseling is that the fluid rapidly darkens with the production of copious amounts

of sub-micron particles, most of which are carbon. Although these particles are too small (and normally

too soft) to cause significant wear, they do have an adverse effect on other fluid properties such as,

foaming, air release and volume resistivity. They can also blind filters.

The effects of static discharge on the composition of degradation products has not yet been established

for phosphate esters but is anticipated to be similar to the effects of micro-dieseling.

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TABLE 2 The Results of Dissolved Gas Analysis by ASTM D831 on Samples Taken from EHC Systems

where Dieseling is Taking Place

Gas Turbine A Turbine B Turbine C Turbine D Fresh Fluid

Oxygen 400 7587 11811 16155 200095

Nitrogen 44200 38868 38493 38909 37120

Carbon Monoxide 10100 195 582 937 ND

Carbon Dioxide 13950 801 3366 4594 651

Hydrogen 1330 55 126 175 ND

Acetylene 1800 128 626 963 ND

Ethane 230 12 55 92 ND

Ethylene 860 64 225 307 ND

Methane 1900 38 213 333 ND

4.3 Thermal Stability and Pyrolysis

The thermal stability of a fluid can provide an approximate guide to its upper operating temperature,

but then only in terms of its ability to withstand breakdown in the absence of air or oxygen (or pyrolysis)

- a situation which is rarely found in practice. In reality, the presence of a small amount of dissolved

oxygen will result in degradation at lower temperatures - particularly in the presence of metals - and

apparent changes in the physical/chemical properties of the fluid may be due to oxidation rather than

pure thermal breakdown.

Several studies have been carried out into the products of pyrolysis and combustion of phosphate

esters. Lhomme et al. [17] examined the degradation products under helium of trimethyl, triethyl, and

triphenyl phosphate and, as a result, proposed a general pyrolysis scheme (Figure 6). Depending on the

phosphate structure and temperature, different degradation pathways were followed. At “low”

temperatures, phosphates followed reaction (a) as a result of the cleavage of the —C—O bond and the

production of olefins, monohydrogen phosphates, dihydrogen phosphates etc. With increasing

temperature, path (b) was followed, involving the breakage of the —P—O bond, while at very high

temperatures, the phosphate residue forms phosphorus pentoxide (probably after passing through an

intermediate phase involving the formation of polyphosphates and pyrophosphates). The pentoxide

rapidly reacts with moisture in the atmosphere, first to form metaphosphoric acid and then phosphoric

acid itself.

With triphenyl phosphate, reaction (b) predominated with initial decomposition not occurring until

>6000C and not being complete at 1000 0C. It seems highly probable that where there is substitution on

the aromatic ring, degradation would initially follow reaction (a), and then (b) when unsubstituted rings

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were formed. It is interesting that both processes produce carbon, a significant contaminant when

dieseling occurs.

FIG. 6 General Pyrolysis Scheme for Phosphates

Pyrolysis studies of IPPP/46 under a helium atmosphere have also been carried out at temperatures

between 500°C and 1000°C (Table 3[15]). Measurements were made of (1) the amounts of carbon

monoxide and dioxide formed using non-dispersive infrared analysis, (2) the organic volatiles, which

were identified using gas chromatography/mass spectrometry, and (3) the amount of phosphorus

pentoxide generated, which was collected in aqueous potassium hydroxide and determined as

orthophosphate by ion chromatography. For comparison, the same product was also examined for the

production of carbon dioxide and phosphorus pentoxide under combustion conditions by passing air

over the sample. Under combustion or oxidative decomposition, the product shows significant

degradation at the latter temperature, but the values for phosphorus pentoxide content were lower

than might be expected.

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TABLE 3 Development of Carbon Oxides and Phosphorus Pentoxide under Pyrolysis and Combustion

Conditions for an IPPP/46 Phosphate

Yield of

degradation

products (%)

Temperature (C)

500 700 1000

Pyrolysis Combustion Pyrolysis Combustion Pyrolysis Combustion

Carbon monoxide 0.63 — 0.56 0.1 1.1 11.23

Carbon dioxide <0.001 — <0.001 0 <0.001 49.63

Phosphorus pentoxide <0.01 — <0.01 0.57 <0.01 1.42

The three main degradation processes are thus responsible for the generation of a wide variety of

degradation products. If these are to be minimised the fluid should be kept clean and dry, and with a

low level of acidity. However, attempting to reduce the level of degradation will be difficult to achieve if

the system design encourages oxidation to take place or allows moisture ingress; or if the fluid has poor

stability.

5 Purification Treatment

The principal behind all conditioning systems is that the rate of acid removal must be greater than the

rate of production. If not, the control of the fluid degradation is lost, the acidity will increase quickly and

fluid life will be considerably shortened. However, purification is not just about the removal of acid but

also about keeping the fluid dry and clean.

5.1 Historical Background

In 1966, Schober [18] reported on the successful continuous treatment of phosphate esters by fuller’s

earth (an attapulgus clay based on aluminosilicates) on a bypass system to the main fluid reservoir of a

steam turbine. Continuous (rather than intermittent) treatment was found to have advantages in

extending the life of the fluid and keeping the fluid dry as a result of water adsorption by the earth. In

addition, continuous treatment used less earth. The possibility of interaction between the earth and the fluid

degradation product was discussed and a pyrophosphate (or a mono-valent metal salt thereof) was

extracted from used earth. The inference was that the aluminosilicate in some way catalyzed the formation

of the pyrophosphate in addition to the conventional removal of acid.

The use of fuller’s earth to remove “active chemical species” from phosphate esters that were causing servo-

valve erosion was reported by Wolfe and Cohen [9]. They too attempted to identify compounds that were

removed from the erosive fluid. In addition to finding chloride present, the magnesium salt of di-m-cresyl

phosphoric acid (Figure 9) was also identified and thought to be a reaction product of the fuller’s earth and

fluid degradation products.

The first reports of the use of fuller’s earth producing deposits in control systems started to surface in

the late 1970’s, primarily in France and Germany. In France, a survey in 1978 [19] suggested that not all

power stations at that time were fitted with fluid purification units, and those that were installed were

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under-sized, i.e. had insufficient fullers earth to control acidity generation. There was also a general lack

of appreciation of the importance of controlling acidity generation. As a result, fluid acid values in

problem stations had risen to 5-10mgKOH/g. Where earth treatment was used, such high acidities were

associated with deposits which were found to contain calcium and magnesium salts.

As an alternative adsorbent, activated alumina was evaluated and found to offer fewer problems in this

respect. Although sodium could be extracted in significant amounts, no deposits were observed. It was,

however, reported that alumina was less effective than fuller’s earth in improving the resistivity, corrosion

and oxidation performance of fresh fluid [20].

In Germany a similar report of system deposits was made by Grupp in 1971 [21]. Analysis of the gelatinous

materials revealed the presence of a variety of metal soaps, some of which arose as a result of system

corrosion and others, e.g. calcium/magnesium, from the fuller’s earth. In order to clarify the origin of the

calcium and magnesium, the composition of earths from different suppliers was investigated and the

presence of calcite (CaCO3) and dolomite (CaCO3.MgCO3) were identified as the sources. It was proposed

that the mono- and di-hydrogenphosphate degradation products reacted chemically with the calcium and

magnesium carbonates to form the metal soaps. Depending on the degree of hydrolysis, the molecular

weight of the salts varied, with the possibility of the production of polymeric materials of high molecular

weight.

As a result of this investigation, the formation of deposits, especially during shutdowns or on other occasions

when the fluid was allowed to cool, was forecast if the total content of calcium and magnesium exceeded

30ppm. It was also recommended to use an earth free of carbonate content. However, the manufacturers

subsequently indicated that, as fullers earth is a natural product, this aspect could not be guaranteed. As

a result, problems of this type continued, and in 1992 Grupp provided additional information on the

sensitivity of clays to the presence of both water and acid [22].

In addition to the problems associated with metal soap formation, several other operating difficulties are

associated with fuller’s earth; in particular the need to dry the earth before use as the presence of water

reduces its efficiency, and also its occasional “lack of activity”. (There have been a number of cases

where fuller’s earth replacement has failed to have any significant impact on acid removal). Normally the

problem arises at “high” acid levels. It could be caused by a high water content or a lower rate of acid removal

in comparison with the rate of formation, but was not investigated in any detail.

As an alternative to fuller’s earth some utilities converted to using activated alumina, a synthetic

product with a known and guaranteed composition. In a few cases the change was made because alumina

seemed to be effective when fuller’s earth failed to reduce acidity levels, and utilities would use alumina

to reduce the acidity to an acceptable level before replacing it with fuller’s earth. A reluctance to use

alumina on a continuous basis can be attributed to a higher price and perhaps also to the fact that the

commercial aluminas had a wider particle size distribution than the earths. There was, therefore, a

greater possibility of “fines” escaping from the cartridge into the system and, being hard particles,

promoting wear. It was necessary – perhaps even more important – to dry the alumina before use as this

solid absorbs more moisture than the fuller’s earth.

16

Most of the experience quoted above has focused on the presence of deposits and their avoidance by using

dry fluid or by changing or modifying the composition of the solid. In contrast little attention was paid

initially to controlling the acidity. Schober [18] and Tersiguel-Alcover [19] suggested that the maximum level

in service should be 1 mg KOH/g. However, it became apparent that even this level was too high to avoid

deposit formation and turbine builders’ recommendations for limits on in-service fluid began to drop,

initially to 0.5 mg KOH/g and then to 0.3 mg KOH/g. A report [23] published in 1979 on the effectiveness

of different types of adsorbents advised maintaining an acidity of <0.2 mg KOH/g, and subsequently even

lower values were suggested, as long as the fluids were treated with adsorbent solids which contained

extractable metals. A low acidity has, of course, the advantage of reducing the sensitivity of the fluid to

hydrolysis as the process is acid-catalyzed, and while the maintenance of low acid levels has an obvious

effect on fluid monitoring and treatment requirements, the slightly increased analytical and labour costs are

normally small in comparison with the cost of a replacement fluid charge.

Molecular sieves have been proposed for conditioning phosphate esters [24], preferably in conjunction

with a coalesce separator to remove water. In the authors’ experience, however, molecular sieves do not

remove partial phosphates from solution and can themselves promote metal soaps formation at

relatively high acidity levels. This technique has not gained widespread acceptance.

The operating problems associated with the use of fullers earth and alumina have led users, turbine builders

and fluid suppliers to investigate other potential conditioning agents, and two separate technologies have

arisen in the last twenty years. The first is the development of a “synthetic” fuller’s earth, also known as a Y

zeolite, in combination with a purer form of activated alumina [25]. The material is very effective at adsorbing

acids but can only be used with fresh fluid or fluid which has not been significantly degraded. When acidity

levels increase it still appears possible to form metal soaps, although the tendency is significantly less than

with “natural” fuller’s earth. As the solid releases water during acid adsorption it is necessary to use a

water-absorbing filter immediately downstream of the solid. Some concern has also been reported

regarding slow improvement of fluid resistivity and the possibility of hard particles of alumina escaping into

the system. The other development has been the introduction of ion-exchange resins and these are

discussed more fully in the following section.

5.2 Ion Exchange Systems

The possibility of using these resins was originally investigated by Wolfe and Whitehead [26] in 1977 in

experiments with strong base anionic resins. Although effective in reducing acidity, this technique was

rejected because of price, the need to use ‘wet’ resins, as well as the fouling of resin beads with gels and

silicone anti-foams. Wolfe and Whitehead were, however, only looked at acidity reduction, rather than

removal of metal soaps, and also used degraded fluid with an initial acidity of 2-3 mg KOH/g - far in excess of

what is typical (or permissible) today.

The next step forward was taken at Gösgen nuclear power station (Siemens 990MW nuclear steam turbine)

in Switzerland in 1983 [27]. As a result of the failure of fuller’s earth to control acidity generation in a

"synthetic" fluid (and because of associated foaming problems and deposit formation on valves), the

possibility of replacing the earth was investigated. Laboratory tests indicated that a weak base anionic resin

17

(WBA) in the hydroxide form could quickly reduce the acidity while a strong acid cationic resin (SAC)

would, surprisingly, reduce the metal soap content. A decision was made to use these resins in

the existing bypass loop in conjunction with a molecular sieve to remove the water released as a result of

the expected exchange mechanism. In order to reduce the amount of water released by the resins they

were dried at 80°C to a level of ~5% before use as a mixture in the existing filter housing. Over a period of

about 1-2 months the properties of the fluid returned to close to those of a new fluid (Figure 7). When the

metal soaps had been reduced to a very low level the treatment reverted to the use of a WBA resin alone.

The fluid continued to operate without problems and in good condition until it was replaced in 2010 after

31 years, not because the product had deteriorated but because the fluid was being withdrawn. This

performance was achieved using intermittent treatment of about 2 days for 2-3 times a year and the resin

charge was replaced every three years [28].

FIG. 7 Change in Fluid Properties at Gösgen Power Station during Early Stages of Resin Treatment

The success of the Gösgen experiment resulted in wider use of the resins in Switzerland and Germany, where

experience was also very positive [29] and prompted a more systematic investigation into their potential [30].

In Canada, the first country outside Europe to adopt the new technology, it was reported that by 2001 a

major gas pipeline company that operated about sixty turbo-compressors on phosphate esters, had

accumulated 6 million operating hours on ion exchange resins. As a result, considerable savings were

being made, mainly on fluid replacement costs [31].

Also in Canada, Ontario Power Generation introduced resin treatment in 1993 and the subsequent

operating experience has been well documented [32, 33, 34] while information on its use in the USA is

reported in reference [35].

Use of this technology has now spread to many utilities in Europe, the USA and parts of the Far East. Some

changes have, however, been made since the initial trial. For example, where cationic and anionic resins are

used they are now mainly contained in cartridges and kept separate from one another.

18

While most users initially tried to use wet resins (typically 50-65% water), not all were successful. The need to dry

the fluid after ion exchange treatment (perhaps using vacuum dehydration) to reduce the amount of water

present encouraged the development and use of ‘dry’ resins, normally containing less then 10% water. It was

also realized that while WBA resins were useful in many applications, there were contaminants that strong base

anionic (SBA) or strong acid cationic (SAC) resins responded better to. Various resin type mixtures have also been

proposed depending on the fluid composition.

5.3 Resin Composition and Structure

Chemically, ion exchange resins are based on a divinylbenzene or polystyrene core onto which different

functional groups can be introduced. These functional groups can react with other chemicals leading

either to an exchange of ions or to the adsorption of the chemical onto the resin surface. As a result,

unlike Fuller’s Earth or Selexsorb, they can remove different degradation products or contaminants from

solution. Function of commercially available resin types is described in Table 4 [36].

TABLE 4 Function of Commercially Available Resin Types

Resin Type Benefits Disadvantages

Weak Base Anion Removes strong acids and metal soaps/salts by adsorption

Weak acids remain

Strong Base Anion Removes weak acids Strong acids remain

Weak Acid Cationic Will remove neutral soaps Releases acid into the fluid Less thermally stable

Large volume changes on going from H

+ to M

+ form

Strong Acid Cationic Will split neutral soaps and remove metals

Releases acid into solution. Need to use chloride-free version of

resin

Chelating Remove acid rapidly May release sodium into the fluid with possible adverse effect on soap

precipitation and foaming properties.

In general, there are three different types of resin structures where the polymer matrix differs:

Gel type. These resins are translucent and the pore structure depends on the degree of crosslinking (or amount of divinylbenzene - DVB) which is usually in the region of 8-10%. A potential disadvantage of this type is the large change in volume as the ionic form changes and the small pore size results in a longer time to reach exchange equilibrium. A gel resin was one of the resins first used by Göesgen power station for removing the metal soaps by adsorption.

Macroporous resins are opaque and contain as much as 20% DVB (i.e., much more cross-linking than gel types and are processed to produce much larger pores than are found in gel types). Due to the higher crosslinking these resins are less sensitive to the effects of drying (i.e., show less shrinkage) and react faster. They also have better strength than gel resins. A particular form of macroporous (or macroreticular) resin is the so-called chelating resin. This type of resin has a functional group that has a greater selectivity for certain metal ions. For example the amino-phosphonic group will preferentially remove calcium or magnesium if there is a mixture of ions

19

present.

Macronet resins have an even higher internal surface area and are rigid (i.e., they do not shrink

significantly). The exchange capacity is, however, much reduced. This may not be critical if the

mode of action is by adsorption rather than by ion exchange.

As indicated earlier, one of the main differences between the original application of the IX resin and those

currently commercially available is that originally the resin was used in bulk. Today, almost all resin use is

in cartridge form and their designs vary, some of them having axial, others radial, flow. Unless the flow

rate through the medium is very slow, axial flow is normally preferred. The resin type is not usually

indicated although it appears that most suppliers offer WBA resins. However some suppliers also provide a

combination of WBA and SAC resin or SBA and SAC.

When examined, all resins, both new and used, showed a spherical bead structure. Most of the spent

resins were coated with insolubles to various extents depending on the EHC system design and the

primary degradation mechanism. Water washing is not always able to remove the coating. Figure 9 [37]

shows magnified images of typical new and used resins from EHC purification systems at CANDU stations.

FIG. 9 Magnified Images (40X) of Typical New WBA (left) and Two Different Used Resins (middle,

right)

Usually, in-service fire-resistant fluids are dark, indicating the presence of large amount of insolubles.

Membrane Patch Colorimetry tests (ASTM D7843-12) carried out on used fluid samples gave results in the

range of 66 to 77 CIE dE), and although typical ISO Cleanliness Codes were 17/15/12 or better, significant

amounts of insolubles - particularly in the range of 0.45 to 4 microns - were found.

In stations with a predominately dieseling problem, the control of fluid acidity is difficult. This is a result of

a combination of the impact of system design, inadequate capability of the purification system and poor

maintenance. The acid number trends in four different turbines are presented in Figure 10.

20

FIG. 10 Acid Number Trends in ABB Turbines [with Permission from Ontario Power Generation]

Currently, different resin treatment systems for EHC fire-resistant fluids are commercially available.

However, if end users do not identify the primary degradation mechanisms and the nature of the fluid

contaminants in their EHC systems, the selected IX system may not provide the expected results. Due to

limited publications on the application of IX treatment some users perform their own studies to obtain

more details on available options for this treatment.

5.4 Evaluation of Different Resins

A used fluid based on a mixture of isopropylphenyl phosphate and trixylyl phosphate fire-resistant

phosphate ester was obtained from an EHC system without servo valves, and was chosen to investigate

the ability of different resin types to reduce fluid acidity. The condition of the fluid is presented in Table

5 [37].

TABLE 5 Analytical Results of Used FRF Fluid before Test

Analysis Units Specification Actual Results

Acid Number mg KOH/g <0.2 0.26

Volume Resistivity @ 20˚C Gohm-cm >4 3.84

Moisture Content mg/kg <800 616

Particle count in range 5 – 10 μm 8646

Particle count in range 10 – 25 μm 3075

Particle count in range 25 – 50 μm 160

Particle count in range 50 – 100 μm 19

Particle count in range > 100 μm 0

ISO 4 Cleanliness Code 17 16

ISO 6 Cleanliness Code 15 14

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Mar-

05

Jul-05

Nov-0

5

Mar-

06

Jul-06

Nov-0

6

Mar-

07

Jul-07

Nov-0

7

Mar-

08

Jul-08

Nov-0

8

Mar-

09

Jul-09

Nov-0

9

Mar-

10

[m

g K

OH

/g]

Acid Number (V20, IX Rig Inlet)

U1 V20

U2 V20

U3 V20

U4 V20

Limit

21

ISO 14 Cleanliness Code 12 10

ASTM Colour ASTM Scale <5 8

Air Release minutes <8 7.2

Varnish Tendency (MPS) CIE dE <30 95.1

Foam Tendency mL 25 <400

Foam Stability mL <10 <10

Weight of deposit after filtration through 5 μm filter

g Not specified 0.0135

Weight of deposit after filtration through 0.45 μm filter

g Not specified 0.0815

Concentration of Al ppm <2 3

Concentration of Na ppm <30 5

Concentration of Ba ppm <2 2

Concentration of Li ppm <2 17

Concentration of Cr, Cu, Fe, Pb, Ni, Sn, B, Zn, P, Mg, Ca, Mo, Ag, Si, Ti and V were below 1 ppm

Ten commercially available and three developmental ion exchange resins provided by seven different

manufacturers were selected for testing (see Table 6). Two resins were received in dry form and all

others were dried in the lab by heating them at 60˚C under vacuum until constant weight was obtained.

TABLE 6 Characterizations of IX Resins Received from Supplier

Supplier Resin Resin Type Density [g/mL]

Water Retention [%]

Supplier I A WBA (macroporous) 0.6191 49.3

Supplier I B WBA (macroporous) 0.5749 52

Supplier I C WBA (macroporous) 0.6503 54.9

Supplier II D WBA (macroporous) 0.4374 4.4

Supplier III E SBA (gel type) 0.68 60-65

Supplier III F WAC (macroporous) 1.1 55-60

Supplier III G SAC (macroporous) 0.74 56-60

Supplier III H WBA (macroporous) 0.62 54-60

Supplier IV I SBA (gel type) 0.64 55-65

Supplier V J SBA (gel type) 0.69 54-60

Supplier VI K SBA (macroporous) 0.425 61.01

Supplier VI L SBA (macroporous) 0.6275 61.51

Supplier VII M SBA (macroporous) 0.4908 1.8 WBA – Weak Base Anion, WAC – Weak Acid Cation, SBA- Strong Base Anion, SAC – Strong Acid Cation

Nine resins marked from A to F and H, L & M were tested separately. In addition, four combinations of

resins were also evaluated. These were: G&B, G&I, G&J, and G&K. They were tested using 3:2 cation to

anion ratios.

The test was carried out by passing 3.8 liters of fire-resistant fluid through a glass column containing 200

mL of IX resin in a recirculation mode for 168 hours. The arrangement is shown on Figure 11. It includes

a condenser, a moisture guard to protect against ingress of atmospheric moisture, and a sampling port.

The temperature was maintained at 40±4˚C. The flow rate was maintained at 17 mL/min using a

peristaltic pump. This meant that it took approximately 4 hours for a single pass of the entire fluid

through the column. A sample of 50 mL was removed after 4, 24, 48, 120, 144 and 168 hours for testing.

22

FIG. 11 Experimental Setup for IX Column Treatment

Test results are presented in Figure 12 [37].

FIG. 12 Acid Number after Treatment by Different Resins

Users operating EHC systems with servo valves also need to control volume resistivity to prevent

electrochemical corrosion of servo valve spools. Since in the past there were some concerns regarding

ion exchange resin ability to control this property, fluid samples obtained during the IX treatment test

0

0.05

0.1

0.15

0.2

0.25

0.3

0 20 40 60 80 100 120 140 160 180

Aci

d N

um

be

r [m

g K

OH

/g]

Hours of Filtration

Acid Number after Treatment by Different IX Resins

A (Wet) A(Dry) B (Wet) B (Dry) D (Dry)

M (Dry) C (Dry) F(Dry) E (Dry) H (Dry)

L (Dry) G&J (Dry) G&I (Dry) G&B (Dry) G&K (Dry)

23

were also tested for volume resistivity. Results are presented in Figure 13 [37]. It should be noted that

these results were obtained on fluids with a high insoluble content and that, as yet, the effect of the

insolubles on the efficiency of acid removal and increase in resistivity have not been clarified. However,

it is probable that the activity of the resin will be adversely affected by fouling of its surface.

FIG. 13 Volume Resistivity at 20˚C after Treatment by Different Resins

To support selection of the best resin type for the IX treatment additional qualification tests were

performed. These tests involved assessment of crush strength (friability), the percentage of perfect

whole beads (PWB), the content of fines and amount of extractable material (inorganic and organic).

The results of the resin qualification tests can be summarized as follows:

Crush strength of the six resins listed above varied in the order: D (dry)> H (dry)> J (dry)> C

(dry)> G (dry)> M (dry). Resin qualification tests were not conducted on “I” dry since it had listed

properties that were similar to “J” dry. M dry resin failed to meet the 3N/bead crush strength

criteria typically expected of nuclear grade resins used at CANDU stations. Hence additional

qualification tests were not carried out on this resin. All of the other resins met or exceeded the

crush strength criteria.

All of the resins that passed the crush strength criteria also showed good perfect whole bead

(PWB) values in the range 96-99.5% and very low fines (<0.1%) content.

The water extractable residue from the various resins varied in the order: G (dry)>J (dry)>C

(dry)>D (dry)=H (dry).

0

5

10

15

20

25

0 20 40 60 80 100 120 140 160 180

Vo

lum

e R

esi

stiv

ity

[Gig

oh

m-c

m]

Hours of Filtration

Volume Resitivity after Treatment by Different IX Resins

A (Wet) A(Dry) B (Wet) B (Dry) D (Dry)

M (Dry) C (Dry) F(Dry) E (Dry) H (Dry)

L (Dry) G&J (Dry) G&I (Dry) G&B (Dry) G&K (Dry)

24

Taken into account the ability of the resin to reduce fluid acidity and to improve volume resistivity as

well as displaying good crush strength, a high percentage of perfect whole beads with a low content of

fines and extractables, the preferred resin for EHC systems with servo valves is the WBA macroporous

resin, ‘H’ (dry) followed by another WBA macroporous resin, ‘D’ (dry). However, it was apparent that

not all WBA resins have the same ability to control fluid acidity and resistivity as well as possessing good

physical/chemical characteristics.

To compare the difference in performance of the most active resins in this investigation with a resin

already in use, two additional plots were prepared. Figure 14 compares the effect on acid number

reduction after treatment by two resins currently used in EHC systems in Canada (A in wet and dry form)

and the performance of the two new recommended resins (D&H in dry form).

FIG. 14 Acid Number Trends after Treatment by Existing (A in Wet and Dry Form) WBA Resins and

Two New Recommended WBA Resins (D&H in Dry Form)

Figure 15 compares similar effect on volume resistivity reduction between the current (A wet & dry)

and the new resins (D&H in dry form).

0

0.05

0.1

0.15

0.2

0.25

0.3

0 50 100 150 200

Aci

d N

um

be

r [m

g K

OH

/g]

Hours of Filtration

Acid Number after Treatment by Different IX Resins

A Wet

A Dry

D Dry

H Dry

25

FIG. 15 Volume Resistivity Trends after Treatment by Existing WBA Resins (‘A’ in Wet and Dry Form)

and Two New Recommended WBA Resins (‘D’ and ‘H’ in Dry Form)

5.5 Control of Fluid Moisture

A separate test was also carried out to evaluate three different techniques for removal of moisture from

EHC fire-resistant fluids. These include the active head space membrane, vacuum dehydration and

moisture absorbent media.

The active head space membrane dryer utilizes a hollow fiber-based membrane dryer to generate dry air

continuously by separating moisture from a compressed air source. While this technology is not suitable

for removal of large amounts of dissolved water in phosphate ester fluids in a very short duration of

time it is, however, an excellent option for the cost-effective and continuous removal of moisture from

the FRF fluid reservoir and can also be used on an ‘as required’ basis (i.e. seasonal use, such as in

summer when the relative humidity is higher as compared to during winter).

In order to evaluate this system, four parameters were examined: i.e. high and low relative air humidity

(%RH) and high and low moisture content in the FRF fluid. A schematic of the experimental set up used

to evaluate the active head space membrane dryer technology is shown in Figure 16.

0

2

4

6

8

10

12

14

0 50 100 150 200

Vo

lum

Re

sist

ivit

y [G

oh

m-c

m]

Hours of Filtration

Volume Resistivity after Treatment by Different IX Resins

A Wet

A Dry

D Dry

H Dry

26

FIG. 16 Schematic of the Membrane Dryer [with Permission from Kinectrics Inc.]

The used fluid tested had a moisture content of approximately 600 ppm. Additional moisture was added

to the fluid in order to achieve a moisture content of approximately 5500 ppm. The results of these tests

are summarized in Table 7 below.

As expected, application of air with a lower relative humidity was more effective in removing moisture.

TABLE 7 Results from the Membrane Dryer Tests

Test Initial Moisture Content in FRF

Fluid (ppm)

Relative Humidity, RH of

air (%)

Moisture Content in Reservoir after 1 week

(ppm)

Moisture Removal (%)

1 646 ~0 218 66.3

2 612 33.0±0.4 586 4.2

3 5550 ~0 1000 82.0

4 5550 33.0±0.4 1420 74.4

To evaluate the vacuum dehydration technology another test rig was applied as shown in Figure 17.

FIG. 17 The Vacuum Dehydration Test Rig

Clive Morton, Kinectrics Inc. 01/02/2013 11:26 AM FRF Flow Diagram_R1.dwg

Sparger

V3

Humid Air

Supply

(For Setting Humidity)

V4

V1

Mass Flow

Controller

0-200 sccmRH

RH Probe

Vaisala HM132

KIN-2015425-0

Heated and Insulated

Oil Temperature ~ 45 °C

Drain

Valve

Sampling

Syringe Thermometer

Traceable

Model 23609-176

SN. 7250950

Membrane

Dryer

Balston 76-01

9 LPM Output @ 60 psig

One way

Valve

Compressed

Air

~ 80 - 100

psig

V5

Moist Air

Vent

Molecular

Sieve Trap

3A

Set to 200 sccm Air

COG - FRF Project - N415037

Simplified Flow Loop

7.6 Liter

Steel Oil Reservoir

with Level Gauge McMaster-Carr #1149K14

V2 V6 V7 Alicat

Model MC-200 SCCM

SN. 56408Oil Volume

5.9 litre

27

Approximately 205 L of used fluid was circulated continuously through a commercial vacuum

dehydration and filtration unit. Flow rate was 22.7 L/min and the fluid temperature was kept at 40 0C.

The circulation was first carried out using the coarse (>5µm) filter below. This system is very efficient at

removing moisture.

TABLE 8 Results from the Vacuum Dehydration and Filtration Treatment

Water content (ppm)

FRF Fluid Treated As-Received Fluid Treatment for 18 hours

using Coarse Filter

Treatment for 12 hours

using Fine Filter

Used Fluid 648 152 134

Other tests were carried out with various zeolite-based molecular sieves and superabsorbent polymer-

based beads to assess moisture removal efficacy from FRF fluids. A 1:100 ratio of adsorbent to fluid was

used in these tests. The media tested included:

Molecular Sieves – 3A, 4A and 9A. These molecular sieves had pore size of 3, 4 and 9 A0 respectively.

Super absorbent beads – SA1 and SA2. SA1 is a polyacrylate-based super absorbent polymer (SAP) and SA2 is a blend of organo clay and a polyacrylate-based SAP.

The solution containing the FRF fluid and the treatment product were continuously stirred for a period

of 1 and 60 minutes respectively. The treatment media was then separated from the FRF fluid via

filtration through a 65 mesh screen. The filtrate was then analyzed for moisture, resistivity, acid number

and particle count. Results of the shakeout tests with a higher contact/stirring time of 60 minutes are

shown in Table 9 below.

TABLE 9 Results from Molecular Sieve and Super Absorbent Treatments

Treatment

Media

Contact

Time

[min]

Weight

of Fluid

(g)

Weight

of Mole.

Sieve (g)

Water

Content

(ppm)

Moisture

Removal

(%)

Acid

Number

(mg KOH/g)

Resistivity

(GΩ-cm)

None NA NA NA 648 - 0.31 3.5

3A 60 102.81 1.57 577 11.0 0.22 2.5

4A 60 125.11 1.72 587 9.4 0.23 2.1

9A 60 98.57 1.18 580 10.5 0.22 1.3

SA1 60 97.74 1.20 597 7.9 0.19 0.9

SA2 60 102.03 1.19 579 10.6 0.20 1.4

28

Utilization of molecular sieves and SAPs for treatment of the FRF fluid resulted in limited improvement

in moisture (5-17%) as well as acid number. However, the disadvantage of using these media is the

increased particle count and reduction in volume resistivity.

In summary, a vacuum dehydration unit or active head space membrane dryer are the best options for

removing moisture from EHC fluids.

5.6 Particle Filtration

As indicated earlier there is significant contamination in the EHC fluids used at two different CANDU

stations and the nature of these particles is the subject of an on-going investigation. It appears that the

existing ISO Cleanliness Code (which, for the majority of EHC fluids is at or below 17/15/12) does not

reflect the level of the very small particles present. To evaluate the concentration of particles below 4

microns (the smallest particles measured by the ISO standard), two different methods were used.

The first method filtered the fluid samples first through a 5µm filter, the filtrate then being filtered

through a 0.45 µm nylon filter. After each filtration the patches were dried before weighing. The results

are presented in Table 10.

TABLE 10 Results of Filtration Tests with Coarse and Fine Filters

Fluid Source Weight of patch after filtration of 50 mL through:

5µm Filter 0.45µm Filter

Fluid from EHC system without servo valves

0.0136 0.0815

Fluid from EHC system with servo valves

0.0909 To high to measure

The second method utilized an automatic particle counter in which particles moving through a flow-cell

are back-lit with a high-output light. The particle images are collected and then analysed by computer.

The method is claimed to be able to analyse and size particles as small as 0.7 microns. The results are

presented in Figure 18.

29

FIG. 18 Particle Size Distribution in the Range 0.7 to 25 Microns (Red Colour Indicates Station with

Servo-valves while Blue Station is without Servo-valves)

In general, both methods confirmed that the largest concentration of small particles (particularly in the

range 1 to 4 microns) is at the station with servo valves which is also experiencing a problem with micro-

dieseling.

A well–known process for removing small particles is electrostatic filtration. This involves applying a

potential across two parallel plate electrodes between which the fluid is flowing. Electrophoresis

(removal of naturally charged particles by an electrostatic force) and dielectrophoresis (removal of a

charge-neutral particle by gradient voltage field force) takes place and the particles are collected

adjacent to the electrode.

Two different used EHC fluids (from systems with and without servo valves) were treated by the

electrostatic filtration unit. The results of electrostatic treatment are presented in Table 11. In each

case, approximately 3L of fluid was circulated through the electrostatic filter at 0.8L/min and at ambient

temperature of approximately 230C for about 24 hours. The treated fluid was then measured for particle

size distribution. No patch weights were measured.

TABLE 11 Particle Concentrations Before and After Electrostatic Filtration for only 24 Hours

Station/Fluid Particle Count Number of particles ISO Cleanliness Code

Used fluid from station with servo valves - as received

5 – 10 μm 44770

18/17/13 10 - 25 μm 14930

25 – 50 μm 882

50 - 100 μm 126

30

>100 μm 2

Used fluid from station with servo valves - post electrostatic treatment for 24 hours

5 – 10 μm 35910

18/16/12

10 - 25 μm 12220

25 – 50 μm 683

50 - 100 μm 90

>100 μm 1

Used fluid from station without servo valves - as received

5 – 10 μm 18370

17/15/12

10 - 25 μm 6015

25 – 50 μm 358

50 - 100 μm 52

>100 μm 1

Used fluid from station without servo valves - post electrostatic treatment for 24 hours

5 – 10 μm 10950

16/15/11

10 - 25 μm 3955

25 – 50 μm 182

50 - 100 μm 18

>100 μm 0

It is important to note that these tests were done with a small scale unit, for limited test duration and

under conditions that were not fully optimized. Further improvements in both the particle count and

reduction in patch weight are achievable with an optimized system and longer treatment time.

There are certain advantages in performing such extensive solid particle removal. Firstly, removal of the

small particles will minimize the possibility of particle agglomeration and subsequent plugging of filter

elements or accumulating on critical parts of the EHC system. Secondly, removal of small particles can

reduce the fouling effect of covering the external resin surfaces and thus significantly improve the

efficiency of resin treatment and resin life. Finally, removal of the submicron particles will also improve

the fluid colour.

Large scale tests were performed using an optimized treatment process consisting of electrostatic

filtration, moisture removal (by active head space dryer) and ion exchange resins. This resulted in

significant improvement to fluid characteristics, including colour as shown in Figure 19.

FIG. 19 Colour Improvement of EHC Fluid after Mechanical and Electrostatic Filtration, Treatment

with IX Resins, and Moisture Removal

31

6 Further Work

Although new information was obtained from the presented study, the need for further work was also

identified. First of all, the discussed fluid degradation mechanisms produce different acids, some of

them weak and some strong. Since different ion exchange resins have their preference to absorb one or

the other acid type, it is essential to determine the amount of both strong and weak acids before

selecting resin type for purification treatment. The majority of the current fluid specifications only

require monitoring total acid number.

Another area of concern is a large concentration of small particles - particularly below 4 microns - that

arise in systems where high temperature degradation has taken place. Due to the current limitation in

describing industrial hydraulic fluid cleanliness levels, the concentration of solid particles below 4

microns is unknown. Their presence, however, may significantly influence the efficiency of existing

purification treatment and overall EHC system reliability. Application of existing methodology for

determining particle distribution from 0.7 microns would significantly improve the knowledge and

selection of the method for the particle removal process. Due to limitations of the optical method for

determination of particle distribution below 0.7 microns, there is a need for applying other methods to

evaluate particles in the range between 0.1 and 0.7 microns.

In addition, it is important to determine if the resin is releasing any contaminants into the fluid to avoid

a repetition of the problems identified with fuller’s earth and activated alumina. Current deposit

identification suggests that there is a difference between commercially available resins of the same type

and some of them may result in the release of contaminants into the fluid.

Finally, the interval of ion exchange replacement requires further review. This will be a function of the

stress level imposed by the EHC system design, operating conditions and maintenance capability. In

addition, it is critical to determine the required resin volume to maintain an efficient and long term

operation between the resin replacement cycles. All these factors will have a significant impact on both

the EHC system reliability and operating cost.

7 Conclusions

Ion exchange resins have become a preferred form of treatment for fire-resistant fluids used in EHC

systems. This treatment appears to be more efficient than other media in removing acid and can

remove other fluid contaminants, for example metal soaps and carbon particles. There are, however,

several issues related to resin selection (e.g. which type is most appropriate) and associated purification

techniques for moisture control and particle removal, which require further attention. In order to

ensure that the most effective combination of techniques is used, utilities should attempt to identify the

level/type of contaminants occurring in the EHC fluid and as a consequence, the main degradation

mechanisms.

Fluid deterioration is usually a manifestation of system design problems, operation outside the design

envelope or inadequate maintenance. Therefore improving system reliability must be done with the

co-operation and active engagement of all stakeholders, including engineering, chemistry, operation and

maintenance.

32

Acknowledgment

The authors would like to express their appreciation to the CANDU Owners Group for their permission

to share the results of the investigation to date.

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