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Selection Parameters for Vacuum Oil

Purifiers

WHITE PAPER © 2017 AGC Refining & Filtration LLC

AGC REFINING & FILTRATION

SELECTION PARAMETERS FOR VACUUM OIL PURIFIERS 2

Contents Introduction 3

Types of Contaminants 3

Design Flow Rate of the Purifier 4

Rate of Removal of Contaminants 5

Contaminant Removal Efficiency (Efficiency of Purification) 5

Cost of a Purifier 9

Trade-offs 10

Drying out of Transformer Insulation 12

References 13

Appendix 1: Sample Systems 14



Introduction

The proper selection of a vacuum purifier for oil—whether it is for transformer oil, lubrication oil, hydraulic

fluid, or thermal oil—involves the consideration of many factors. Yet strangely enough, most buyers of oil

purifiers spend relatively little time on the selection process. Since these systems usually involve high

capital cost, it is important to understand the different types and what capabilities and limitations they

possess. The only objective of purification is re-use; and with proper procedures, oil can last indefinitely.

What then are the parameters to consider when purchasing a vacuum oil purifier?

Type of contaminants to be removed by the purifier

Flow rate of the purifier

The efficiency or rate of removal of contaminants by the purifier

The cost of the purifier

Trade-offs

Types of Contaminants

The most common contaminants that must be removed from transformer oil include:

moisture (from condensation or cooling water leakage, etc.),

gases (from leaking compressor gas seals, etc.),

solids (from dust, corrosion, bacterial growth, etc.), and

acidity (from hydrolysis, etc.).

Oil purification and re-use has proven to make good economic sense. Cost of new oil is rising as is the

expense of disposing of contaminated oil. Before considering the economics of oil purification, you must

know what the contaminants are that must be removed before the oil can be re-used. To find out what the

predominant contaminants are, take several samples over a period of time and have them analyzed.

Typical analyses performed on the samples are:

particle size distribution,

moisture,

dissolved gases,

acidity,

viscosity, and

flash point.

Since the rates of contamination and the individual quantities often vary widely, this data is necessary for

the manufacturer to design of an effective purifier. Standard “off-the-shelf” systems often cannot provide

satisfactory results.



Design Flow Rate of the Purifier

Determining the capacity or flow rate of an oil purifier depends on:

the rate at which the contaminants enter into the main oil reservoir and

the required efficiency (i.e. how fast does the purifier need to purify the oil to stay ahead of the incoming contamination).

Figure 1: General Schematic of the Oil Purification Process

Contamination of a volume of oil, whether inside a transformer or in a compressor lube oil reservoir,

occurs at varying rates. Whether qualitatively (moisture, solids, or gases) or quantitatively (in ppm or

percent), it is important to get some idea of what the rate of contamination is, before determining the

capacity of the purifier. In other words, a 1000-gallon reservoir of dirty oil does not necessarily need a

purifier with a 1,000 gallon per hour flow capacity if the rate of contamination entry is small and relatively

constant.

The reason for this is that a purifier with a capacity of 1,000 gallons per hour does not necessarily mean

that this 1,000 gallon volume of oil is clean after going through the purifier for an hour. This is because a

purifier has a certain fixed efficiency of purification. This means that during each pass of 1,000 gallons per

hour through the purifier, only a certain amount of contaminants is removed. This is the so-called

“multiple-pass” purification process.

In the case of a vacuum purifier, the amount of contaminants removed depends on the depth of vacuum

and the amount of heat applied to the oil during its passage through the purifier, as well as the initial

concentrations, types and partial pressures of those contaminants (more on removal efficiency later).

However, while most vacuum purifiers are designed for partial but progressive removal of contaminants



during several passes through the equipment, a purifier can be designed to remove almost all known

contaminants in a single pass through the purification system. This is the so-called “single-pass”

purification system.

Rate of Removal of Contaminants

The removal efficiency of a purifier is defined as the amount of contaminants removed during each pass

through the system.

The length of time it takes to purify the entire volume of oil in a reservoir is therefore a function of the

removal efficiency of the purifier. The higher the efficiency, the faster the contaminants are removed.

If the oil reservoir holds 1,000 gallons and the purifier capacity is 200 gallons per hour, the reservoir will

be turned over in 1,000 ÷ 200 = 5 hours.

This does not mean that all contaminants have been removed.

It just means that after 5 hours, 1,000 gallons has been pumped through the purifier and contaminants

have been partially removed.

Most purifiers are designed for partial removal of contaminants during each pass through the purifier. In

the case of moisture, if the initial total water (free, emulsified & dissolved) concentration is 200 wppm and

each pass through the purifier removes 50 wppm, if should take four passes through the purifier to

remove all the moisture. But, what comes out of the purifier (50 ppm moisture) is returned to the reservoir

where the oil still contains 200 wppm of moisture.

Thus, during each pass that removes 50 ppm of moisture, the oil is returned to a reservoir of oil that has

an ever decreasing moisture concentration. The purification system will eventually purify the entire

volume of the reservoir, if it operates continuously and contaminant removal stays ahead of the incoming

contamination rate. Most purifiers are designed for continuous “kidney-loop” operation. (See Figure 1

above)

This brings us to the parameter of purifier efficiency.

Contaminant Removal Efficiency (Efficiency of Purification)

As stated earlier the efficiency of purification or the degree to which contaminants are removed during

each pass through the purifier depends on:

the temperature to which the oil is raised during the each pass through the purifier and



the depth of the vacuum in the purifier’s vacuum vessel.

Temperature

At the design flow rate, the purification efficiency depends on the amount of heat transferred to the

contaminated oil during each pass through the purifier. The heat must be supplied by special low watt-

density in-line heaters that prevent localized hot spots that could carbonize the oil, foul the elements with

deposits and reduce the heating capacity.

For instance, if the temperature of the contaminated oil in the tank is 70° F and assuming that at the

design flow the heat supplied by the heaters during each pass is 70° F, then during each pass through

the purifier, the oil passing through the heaters increases in temperature from 70° F to 140° F. This is the

normally close to the temperature for the removal of contaminants. The oil is then returned to the tank,

which is still at 70° F. This continues during several passes, until the temperature in the entire tank is at

140° F.

Vacuum

The vacuum system is the heart of the vacuum purification process.

In addition to applied heat, it is the second most critical parameter in the purification of oil and the one

most often neglected by buyers.

The reason for the importance of vacuum is: the degree to which contaminants are removed during each

pass through the purifier is directly related to the vacuum applied to the oil during the purification process.

(See Figure 2)

Furthermore, the depth and reliability of the vacuum system is directly related to the capacity and quality

of the vacuum pump. Of course the quality and type of the vacuum pump is directly related to its cost,

which is the reason many manufacturers substitute less expensive, lower quality and less reliable vacuum

pumps. (See the section on trade-offs)

Since most purchases are price-driven, buyers seldom bother to find out the difference in quality of the

vacuum pumps presented in competitor’s equipment.

Mass Transfer

Another reason why vacuum and temperature are the most important parameters for a purification system

is that both work together to create the mass transfer conditions necessary to vaporize and remove the

contaminants such as moisture and dissolved gases.



Figure 2: Effect of Vacuum on Purifier Performance

The process of vacuum purification is based on the principle of vacuum distillation, a common chemical

engineering process used to split crude oil into fractions.

In a vessel that is placed under vacuum, the oil is dispersed in a thin film, over a large surface area (such

as distillation trays or Rashig Rings™). By applying both heat and vacuum, the contaminants are

vaporized from this thin film of oil and removed by vacuum, at a rate based on their lowered boiling

points.

Like any physical process, this removal is dependent on vacuum, temperature and residence time (the

time the oil spends flowing over the trays or rings in a thin film while exposed to a high vacuum).

Some manufacturers use coalescer elements in their vacuum vessels, the assumption being that these

will separate moisture from the oil under a vacuum. This concept has several drawbacks, not in the least

are the removal of additives and the creation of a difficult to control foam in the vacuum vessel. This often



requires the purifier to be shut down for element change-out and cleaning.

In a properly designed purifier the contaminants that have been separated from the oil are sent through

the vacuum pump exhaust and to the atmosphere.

In this part of the process it is essential that before reaching the vacuum pump the vapor that consists of

moisture and other (corrosive) gases is passed through an evaporator and collected in a condensate

collection tank before it reaches the vacuum pump. The vapor mixture would otherwise corrode the pump

internals, reduce the pump oil viscosity, and ultimately cause failure of the pump. Many manufacturers of

vacuum purifiers neglect this.

In a high-quality vacuum purifier, the vacuum system is the most important and consequently, the most

expensive part.

Figure 3: A Typical Two-stage Vacuum System

In lubrication oil applications, a single-stage vacuum system that produces a vacuum of 1 torr is often

sufficient to remove minor amounts of moisture and gas. These are so-called low-vacuum Systems.

However, to achieve moisture levels below 5 wppm and to remove almost all entrained and dissolved

gases such as for transformer oil, a two-stage vacuum system (see photo) producing a minimum of 10-3

to 10-4 torr is required.



It is possible to design a purification system that will remove all contaminants in a single pass. Power

companies that must purify large oil-filled transformers in a timely manner often need this.

This requires a large system with a high flow rate, a deep vacuum and single-pass purification capacity.

The deep vacuum capacity is also used to dry out the transformer’s cellulose insulation under high

vacuum and to return the dehydrated, purified oil to the transformer, while still under vacuum.

Figure 4: A Large High-Vacuum Transformer Oil Purifier

Cost of a Purifier

The preceding discussion makes it obvious that the cost of a purifier depends on the following:

the design flow rate and

the level of oil cleanliness (removal of contaminants) required.

Thus, the prospective buyer must begin by asking:

“What is my budget,” and

“What can I get for it?”

Then the options available are, (from low cost to high cost, respectively):

1. a low vacuum, low flow rate system (low cost, low removal efficiency requiring many passes),

2. a low vacuum, high flow rate system (moderate cost, low removal efficiency requiring several passes),

3. a high vacuum, low volume system (moderate cost, high removal efficiency requiring one to three passes), and

4. a high vacuum, high volume system. (high cost, high removal efficiency requiring one pass).



Figure 5: Vacuum Levels for Various Types of Vacuum Pumps

Trade-offs

There are many purifiers on the market that claim high contaminant removal rates at relatively high flow

rates.

However, to achieve high contaminant removal rates a system with deep vacuum and high pumping

speed is required. This is only accomplished at a minimum vacuum of 10-3 torr. This level can only be

achieved at high flow rates by a two-stage vacuum system consisting of a blower and a main vacuum

pump.

A low-vacuum pump (1 torr) cannot remove moisture below 10 wppm as is often claimed. (See Figures 2

and 6)

Here a trade-off used by many manufacturers is to use a low-vacuum, rotary-vane-type pump. Besides

the low vacuum, rotary-vane pumps have a relatively low reliability and decreasing efficiency with age

due to the ever-increasing wear of the vanes.

The amount of heat required for purification at high flow rates exceeds the heater capacity on most low-

vacuum purifiers. Thus, it takes much longer to purify the reservoir volume of contaminated oil.

Thus, the claims of removal rates are often exaggerated and can only be verified by careful sampling and

analyses for which most customers do not have the facility or the patience. Allen purifiers provide as an

option an on-stream water-in-oil analyzer to indicate removal capacity in real time.

Low-vacuum and low-efficiency systems will leave most of the contaminants in the oil.



Low-vacuum and high flow rate systems will also leave most contaminants in the oil while taking an

excessive amount of time to reach what little they remove.

High-vacuum and moderate- to high-volume systems will remove almost all contaminants and will often

leave the oil in better condition than when it was new.

Figure 6: Summary of Parameters Influencing the Selection of a Vacuum Oil Purifier

Purifier design flow rate depends on:

the rate at which the contaminant enters the lube oil tank (for instance water or gas leaking from the seals into the oil or moisture entering a transformer through a breather) and

the budgeted cost of the purifier.

Contaminant removal efficiency depends on:

the design flow rate,

the amount of heat supplied to the oil during each pass through the purifier,

the depth of the vacuum applied to the oil, and

the budgeted cost of the purifier.

The number of passes required to achieve purification depends on:

the initial level of contamination,

the amount of heat supplied,



the depth of vacuum supplied, and

the design flow rate of the purifier.

The cost of the purifier depends on:

the design flow rate,

the number of heaters required to achieve the proper purification temperature,

the type of vacuum pump supplied (rotary vane or single- or two-stage lobe-type pump),

the level of purification required,

the amount and sophistication of the instrumentation, control, alarm and safety features (PLC

control, state-of-the-art electronics, etc.), and

additional pre- and post-filtration for solids, acid removal, color restoration, etc.

Drying out of Transformer Insulation

An added benefit of a high-vacuum (two-stage vacuum) system is that it is strong enough to pull a

vacuum on the entire transformer and leave it under vacuum for a number of hours—even days—to dry

out the cellulose insulation inside the transformer. (See also The Aging of Cellulose in Transformers)

A dry ice moisture trap allows the operator to determine when enough moisture has been removed from

the transformer insulation by weighing the amount of ice accumulated in the trap. However, other

methods exist that can gauge the degree of dryness of the cellulose insulation.

After the dehydrated oil is returned to the transformer under vacuum, the remaining moisture in the

cellulose will migrate to the dry oil until a new equilibrium is established between the moisture in the

cellulose and the oil.

Note: The vacuum dehydrator required for purification of lubrication oil such as lube and seal oil for a gas

compressor differs from the one for purification of transformer oil. Hydrocarbon process gas is often

entrained and dissolved in the oil. However, the vacuum required for removal is lower. In addition, the

process gases must be condensed in a refrigerated condensing system before they can reach the single-

stage vacuum pump and cause damage.



References 1. Adams, M.A. & H.P. Bloch. “Vacuum Distillation Methods for Lube Oils Increase Turbomachinery

Reliability” (proceedings of the Seventeenth Turbomachinery Symposium, 1988).

2. “AFI-101, Water Activity in Oil” (internal publication, 2007).

3. “AFI-102, Vacuum Distillation for Industrial Oil” (internal publication, 2006).

4. “AFI-105, The Aging of Cellulose Insulation in Transformers” (internal publication, 2005).

5. “AFI-106, Maintenance of Mineral Transformer Oil, Part 1” (internal publication, 2007).

6. “AFI-107, Deterioration of Transformer Oil, Part 2” (internal publication, 2007).

7. “Aramco ATPR-321-677 Lubrication Oil Sampling Program Results 1980-1985 FCC, CHD units and Wet Gas Compressor Rotating Equipment” (internal report, Saudi Aramco Shedgum NGL Plant, January 1985).

8. “Evaluation of Vacuum Dehydrator Test Units” (internal report, Saudi Aramco Mobil Refinery, Yanbu, Saudi Arabia, December 1993).

9. “Lubrication Oil Recovery using an Allen Oil Conditioner” (internal report, Refineria Isla, Aruba, 1990).



Appendix 1: Sample Systems

Figure 7: An Allen High Capacity Transformer Oil Purifier

1. Air cooler

2. Accumulator tank

3. High-vacuum booster pump

4. Electric hose reel

5. Main vacuum pump

Showing the main vacuum pump and high-vacuum booster pump as well as the air-cooled condensing system



Figure 8: An Allen Transformer Oil Purifier

1. Control panel

2. Automatic vent valve

3. Filter vessel

4. Relief valve

5. Low-watt density in-line heaters

Showing the heaters and a two-element filter vessel



Figure 9: A Transformer Oil Purifier

1. Filter vessel

2. Water-in-oil analyzer read out (optional)

3. Flow meter/totalizer

4. Basket strainer

Showing the flow meter/totalizer and in-line basket strainer

Figure 10: A High-Vacuum Transformer Oil Purifier

For oil volumes of 20,000 gallons and over

www.AGCInternational.com

3045 East Elm Street

Springfield, Missouri 65802, USA

Toll Free: +1 800 865 3208

Phone: +1 417 865 2844

Date post:	17-Jan-2017
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