Water hammer risk evaluation for long distance gravity water transmission

pipelines

Xiaoxue Wang1, Ronghe Wang

2, Jing Luo

3 and Haibo Yang

4

1 Master; Key Clean Production Laboratory, Division of Energy and Environment,

Graduate School at Shenzhen, Tsinghua University; Room 201, Block L, Tsinghua

District, Xili University Town of Shenzhen, Nanshan, Shenzhen City; Email:

932125380@qq.com; 15989503742;

2 Professor, Key Clean Production Laboratory, Division of Energy and Environment,

Graduate School at Shenzhen, Tsinghua University; Room 111B, Block L, Tsinghua

District, Xili University Town of Shenzhen, Nanshan, Shenzhen City; Email:

wang.ronghe@sz.tsinghua.edu.cn; Tel: 0755-26036522;

3 Master; Key Clean Production Laboratory, Division of Energy and Environment,

Graduate School at Shenzhen, Tsinghua University; Room 201, Block L, Tsinghua

District, Xili University Town of Shenzhen, Nanshan, Shenzhen City; Email:

luojing1023@126.com; 15815570137;

4 Master; Key Clean Production Laboratory, Division of Energy and Environment,

Graduate School at Shenzhen, Tsinghua University; Room 201, Block L, Tsinghua

District, Xili University Town of Shenzhen, Nanshan, Shenzhen City; Email:

harryboyang@163.com; 13352998366;

ABSTRACT

Water hammer is of great damage to long distance raw water transmission pipelines.

Once a pipe burst accident occurs, the entire city's water supply will be affected. We

take a city drinking water transmission aqueducts as an example, convert the

construction drawings and general layout plans with AutoCAD format to DXF

format which can be imported to water supply model systems, extract the physical

attributes of the pipes, air valves, control valves, reservoirs and other auxiliary

facilities, establish a computer model of water supply system which contains pipes of

DN2600, DN2400 and DN2200 with a total length of 71km, 127 air valves, 73

control valves, and one reservoir. We verify the model by manually checking the

physical properties of the pipe network with inspecting profiles, charts and tables;

build the water hammer analysis scenarios through adjusting the control valves on

the aqueducts at the outlets; evaluate pipe burst risk by scenario analysis; set a surge

tank at the point of maximum vapor volume to prevent water hammer. The results

show that the air valves cannot prevent water hammer as the water hammer

protective equipment do, and evaluating the water hammer risk for the long distance

water conveyance system is necessary.

KEY WORDS

Water Hammer; Risk Assessment; Model; Water Transmission Mains; Pipe Burst

INTRODUCTION

Water hammer occurs during a sudden operation of valves (open or close in a short

time) and pumps (instant shut down) or an instant flow change, which causes the

phenomenon of liquid column separation and cavitation, and then leads to pipe

wrinkles or breakage and damage to valves and pumps. Research on water hammer

type, intensity, wave calculation, valve operation and mathematical models is a long

history, and recently there comes up pulse test platforms, pulse pipes flushing, and

new simulation methods. By now there is an attempt to simulate and predict pipe

burst based on the instantaneous shock wave theory (Rich, 1963; Wylie & Streeter,

1978; Ghidaoui et al. 2005; Zheng et al. 2009 and Leishear, 2008).

Preventions for water hammer are summarized as：

(1)Lower velocity. This method is rarely applied, because it needs large pipe

diameters and so it becomes expensive.

(2)Use pumps with flywheels or a large moment of inertia. Water hammer will be

reduced while power consumption, wind resistance, and difficulty in driving

increase.

(3)Add a surge tank after pump stations or other key location. It is effective, safe and

reliable, at the same time it brings problems of antifreeze, stagnant water, and high

cost.

(4)Deploy check valves which close slower than the water hammer wave spread back

and forth once.

(5)Deploy check valves which close completely before the backflow of the liquid

column. It works because of a small displacement of backflow and flow rate,

meanwhile a large backflow resistance.

(6) Use air pressure tanks and air valves to adjust the vapor volume in the pipe

(Streeter, 1982; Walski et al., 2001; Shirzad et al., 2011 and Wu et al., 2011).

(7)Remove check valves or deploy balloon-type water hammer eliminators,

pump-stopping water hammer eliminators, self-closing water hammer eliminators,

bypass pipes, bursting disks and so on (Borga, 2000).

A water hammer model was built for a raw water transport network in Sichuan

Province, China, and operation combinations for water hammer risk evaluation were

designed. Water hammer preventions must be evaluated and applied due to great

changes in ground elevation and high pressure in this network. Previous works were

mainly based on pipe ancillary equipment or optimizing operations of pumps and

valves, while few focused on scenario forecast based on hydraulic model. Therefore,

this article takes the latter to simulate, analyze, and respond to possible water

hammer for the network, which will benefit local water groups and make a

demonstration for related works.

EXPERIMENTAL METHODS

Modeling. This paper established hydraulic model and water hammer model of a

city's long-distance gravity pipeline, using steady-state and quasi-dynamic

calculations.

The original data collection. Pipe network modeling is the first to collect the data

required. The accuracy of the raw data determines the accuracy of the model’s

calculate results. The raw data of the water pipelines include pipeline layout,

cross-section diagram, pipeline pressure and major equipment, flow resistance

coefficient, flow coefficient and the valve opening curve and the type and operating

rules of valves. The raw data needed to be aggregated and organized into a format

which model calculation program can read.

Modeling and Verification. After the completion of the data preparation phase, we

imported the data into the database of the network model by manually editing the

data. By DXF graphics conversion we extracted the geographic coordinates, relative

position and physical properties of pipelines and ancillary facilities. And then

manually imported diameter, elevation, water demand, pump stations and valve

operation, etc., and established the correspondence of various types of data (Figure 1).

Because original data may need to be amended over time, attention was paid to the

data correspondence when importing data.

Figure 1.Model data storage and operation

The water pipeline includes two parts, and both are gravity flow pipelines (Figure 2).

Load raw data

Pipeline data

Other data Core data

Model operation parameters

Results data

Model calculation

program

Model database

First part are pipes of DN2600, PCCP are from water source to the city outer ring

road, length of about 26 km, ground elevation of starting is about 75 m higher than

ending point. And there is a flow control station before the pipe into city outer ring

road, with two DN1800 piston valves in parallel controlling flow and pressure,

minimum water level of 603.3 m and highest water level of 608.3 m. The second part

of the pipeline is main water distribution line along city outer ring road, DN2200 and

DN2400 steel pipe, a length of about 30 km, connected with the first part of the

pipelines in the middle. Several nodes along the pipeline supply to the city, the nodal

flow is shown in Figure 2. The total flow of the left branch line is 22208.33 m3/h and

the total flow of the right branch is 7041.67 m3/h, the total amount of water from

reservoir is 35750.00 m3/h.

Model Verification. When modeling was completed, the first stage was checking of

input parameters. Diameter, tube length and elevation were tested compliance with

raw data, in accordance with the order of flow from large to small. And then verified

the location of the various components of the pipeline according to the stake mark

order. Checked valves, drain, air valve, adjustable in different classes and adjusted

the specific valve type and caliber, mode of operation, the head loss coefficient, air

valve orifice size and type of control and so on, to make them consistent with the

reality and ensure the rationality of its operation.

When checking the process of hydraulic calculation, steady-state hydraulic

conditions in the pipeline should be calculated firstly. The propose of setting

operating parameters of the calculation engine is to get preliminary calculation

results, including pressure, flow, vapor pressure and the amount of steam. Then the

results was compared to data initialization, making use of part of flow and pressure

data as a validation set to identify possible errors and omissions in the modeling

process. In this step, due to the huge number of parameters, complex structure and

logical controls in the quasi-dynamic model, it was difficult to use ordinary

automatic parameter determination method. In recent years although parameter

optimization based on genetic algorithm (GA) and reverse hydraulic calculation

method are universal, the method of parameters determination is relatively complex,

in practice, manual check is more common. In accordance with the order of

sensitivity, we adjusted the user's demand, to shorten the gap between flow rate and

pressure and the actual value. Then Through estimating leakage loss according to the

pipe network night minimum flow, we adjusted the pipe network leakage and finally

adjusted wall hydraulic roughness C. Preliminary calculation was repeated to

improve the accuracy of the model after some adjustment.

Water hammer simulation. The total length of pipeline is close to 60 km. The

whole travel time of water hammer wave is close to 2 min. The wave velocity value

in the steel pipe is 1200 m/s as an analog velocity based on experience. In order to

fully understand the spread of water hammer, the end of the calculation time was set

to more than 6 min after valve closed , and t = 0 s was the normal steady flow state.

Considering the length, wave velocity of each tube and complexity of the calculation,

we set the time step of 0.1 s.

Control valve. According to these pipelines, we designed a number of conditions that

may cause water hammer. Based on the actual usage of water pipelines, we set

different operating time of control valves at outlets on main pipes, as control

methods of water hammer analysis. Table 1 is a list of control valves, where “GPV”

means “valve”, followed serial number and position (Stake + distance to Stake). The

distribution of the control valves is shown in Figure 2.

Table 1. Control valve

Valve Number Stake + distance to Stake

GPV-2 113+90

GPV-3 171+88.8

GPV-6 252+0-1NORTH

GPV-7 157+60WEST

GPV-8 126+40

GPV-9 99+50

GPV-12 33+73

GPV-13 11+45

GPV-14 00+90

GPV-15 158+90EAST

GPV-16 243+80

GPV-17 300+45

Figure 2. Map of control valves and Demand

Design of the valve operating time. Different operating time was set according to the

roles and characteristics of different valves, DN1800 piston valve and DN1600

butterfly valve for 30 min, DN2200 and DN2400 butterfly valve for 45 min, DN2600

butterfly valves for 60 min. For the evaluation of the water hammer’s damage in the

case of emergency, there were also 1 min, 6.7 min (400 s) operating time. Then we

formed 12 scenarios (Table 2) through combining control valves and their design

operating time, each including several closing time. For each closing time, several

lines and specified points were calculated simultaneously.

Table 2. Scenario plan list

Number Control valve Diameter (mm) Closing time (min)

1 GPV-6-252+0-1NORTH 1,800 6.7 30 90

2 GPV-2-113+90 2,600 1 6.7 60

3 GPV-12-33+73 1,600 1 6.7 30

4 GPV-3-171+88.8 1,600 1 6.7 30

5 GPV-7-157+60WEST 2,200 1 6.7 45

6 GPV-8-126+40 1,000 1 6.7 30

7 GPV-9-99+50 1,600 1 6.7 30

8 GPV-13-11+45 1,000 1 6.7 30

9 GPV-14-00+90 1,400 1 6.7 30

10 GPV-15-158+90EAST 2,400 1 6.7 45

11 GPV-16-243+80 2,400 1 6.7 45

12 GPV-17-300+45 1,600 1 6.7 30

Pipeline design pressure. Pipes have different bearing capacity of pressure, which

will influence the risk of water hammer. Table 3.

Table 3. Maximum working pressure and maximum test pressure of the

pipelines

No. stake mark

distance

from

reservoir

(km)

material

maximum

working

pressure

(MPa)

maximum

test

pressure

(MPa)

roughness

1 00-65-100+00 10.04 DN2600

(PCCP) 0.4 0.6

Lined with

cement

mortar

2 100+00-162+80 16.41 DN2600

(PCCP) 0.6 0.9

Lined with

cement

mortar

3 162+80-252+53.3 25.00 DN2600

(PCCP) 0.8 1.1

Lined with

cement

mortar

4 A00+00-A00+93.2

(at intersection) 25.52

DN2600

(Steel) 0.9 1.4

Lined with

epoxy resin

5 00+15-157+52

(west) 41.32

DN2200

(Steel) 0.9 1.4

Lined with

epoxy resin

6 157+52-300+65

(east) 39.90

DN2400

(Steel) 0.9 1.4

Lined with

epoxy resin

RESULT ANALYSIS

12 scenarios above were simulated, and the calculated results were summarized in

the pipeline cross-sectional view to show the highest, lowest hydraulic head and

maximum volume of steam of different points. Figure 3 shows that the hydraulic

conditions in a segment of pipeline. X-axis represents the distance from the start

point, y-axis represents hydraulic head, where red is the highest level within the

simulation time, the minimum level blue, and green for the pipe elevation.

Scenario-1: Flow control stationGPV-6-252+0-1NORTH. The flow control station

has two DN1800 pipes in parallel, each have a piston valve to control flow, that

GPV-6-252+0-1 North and GPV-6-252+0-1 South. The hydraulic head changes along

the pipeline caused by the closure is shown in Figure 3, local water flow and pressure

changes over time at Stake 250 +90 approximately 160 m upstream of the flow

control station can be observed in Figure 4. For all the closing time the pipeline is

safe if one control valve is in the fully open state, the other is being closed. But it

was very dangerous if the two piston valves in flow control station closed at the same

time, or one was off, another was closing. In the case of one closed state the other

being closed in, 6.7 min closing time caused a high pressure of 2.3 MPa, far

exceeding the maximum pressure of the pipeline; The result is still not safe when

closing time extended to 30 min, the maximum pressure was 1.2 MPa; In fact,

closing time of piston valve in flow control station was extended to 90 min, it still

caused 0.86 MPa high-pressure upstream 160 m, higher than DN2200 maximum

working pressure there (0.8 MPa). The result is still dangerous.

For these situations, an assumptions protective measure was set in the model: a surge

tank upstream of the flow control station. The calculation results showed that it can

effectively eliminate danger of high pressure. The highest pressure was 0.735 MPa,

lower than the maximum working pressure 0.9 MPa when this valve closed for 6.7

min.

Figure 3. Scenario-1 hydraulic head changes along pipes (one off, another

shutting down)

Figure 4. Partial pressure and flow changes (closing time 6.7 min, 30 min and 90

min)

Scenario-2: GPV-2-113+90. In the view of pipeline from the reservoir to the left

valve GPV-14, closing valve for 60 min did not cause danger. Closing valve for 1

min caused a high-pressure of 300 m water column (2.9 MPa), and 6.7 min for 110 m

(1.08 MPa), far exceeding the pipe’s maximum working pressure, which was

dangerous. In addition, 6.7 min closing time caused 0.49 MPa high pressure 5 km

distance from the reservoir, while the maximum working pressure of pipe here was

only 0.4 MPa.

Scenario-3: GPV-12-33+73. GPV-12-33+73 valve didn’t cause a great impact, as it

was in branch pipe, where the flow rate was only 1950m3/h. The closing time for 1

min only led to 7 m change of hydraulic head at the end of the pipeline. The

maximum pressure of the pipeline at all locations was lower than the maximum

working pressure. It is the same for 6.7 min and 30 min.

Other scenarios. Control valves which did not cause any danger were

GPV-17-300+4, GPV-14-00+90, GPV-13-11+45, GPV-9-99+50 GPV-8-126+40 and

GPV-3-171+88.8, regardless of closing time. These valves in branch line were

Similar to GPV-12-33+73. Closing for 1 min caused a slight pressure changes, but in

a safe range.

In a view from reservoir to the east end of this pipeline, GPV-15-158+90 East and

GPV-16-243+80 closing for 1 min resulted in tiny danger. Closing GPV-16-243+80

resulted in 110 m high pressure upstream of the valve, 54 m higher than pressure of

normal operation. Water hammer reached the middle of the pipeline, upstream the

flow control station after about 50 s, resulting in 20 m fluctuations. Closing

GPV-15-158+90 EAST resulted in 75 m pressure, higher than the normal 35 m. At

stake mark 99 +80 (2 km upstream of GPV-2), 10 km away from reservoir, a highest

pressure reached 0.329 MPa. The pipe here was DN2600 PCCP, maximum working

pressure of 0.4 MPa, close to the highest pressure, so it was dangerous. No danger

occurred downstream of the flow control station, because the highest pressure was

much lower than maximum working pressure, although higher than that of stake

mark 99+80.

Closing GPV-7-157+60WEST resulted in a greater danger. When closing for 1 min,

the maximum pressure generated 12 km from reservoir. Pipe here bore a pressure of

2.04 MPa, far exceeding its maximum working pressure 0.9 MPa and a maximum

test pressure 1.4 MPa (DN2200 steel). In addition, the pressure exceeded the

maximum working pressure of 0.6 MPa upstream of GPV-2-113+90 and 0.8 MPa

upstream of GPV-3-171+88.8, which was dangerous. The results were still dangerous

when closing time was 6.7 min. The pressure was 1.176 MPa downstream of

GPV-7-157+60, higher than maximum working pressure 0.8 MPa of DN2200 steel

pipe there.

CONCLUSION AND RECOMMENDATION

Valves on branch lines didn’t impact much in the water hammer simulation results.

No danger occurred when closing time was more than 1 min. Water hammer was not

serious in pipelines in north and in east because of the small demands. The system

was safe when the valves on main pipe closed more than 6.7 min. The harm in west

was greater than east, as demand in west was larger, but not very serious.

It is reasonable for the first part of pipeline that the design working pressure

gradually increases with elevation decreasing. There is no harm when manual

butterfly valve is closed 45 min (for DN2200 and DN2400) or 60 min (for DN2600),

so it’s reasonable to leave a certain safety coefficient. In addition it avoids many

disadvantages to use combined air valves instead of single-hole exhaust.

The flow control station has a big influence on the system, because of using two

DN1800 piston valves with small diameter and large local loss coefficient. Closing

two valves at the same time caused serious danger according to the simulation, even

for 30 min closing time. The water hammer is the most serious when one valve

closed and the other is closing, the maximum pressure is still higher than working

pressure here even for 90 min closing time. So extending valve closing time cannot

completely eliminate the danger of water hammer. Some water hammer protection

measures are necessary. A surge tank upstream of the flow control station can prevent

water hammer damage caused by piston valves, misuse and failure in pipelines.

ACKNOWLEDGEMENT

The authors acknowledge the financial support of the Shenzhen Fundamental

Research Program of China “Water supply network pipe rupture and leakage control

technology research (JC201105180804A)” and “Water supply network optimization

scheduling technology research (JCYJ20120616213618826)”, and Seventh

Framework Programme (FP7) Marie Curie Actions (PIRSES-GA-2012-318985).

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