Chapter 6

Pressure and Surge Control

MT Stockwater Pipeline Manual

CHAPTER 6 PRESSURE AND SURGE CONTROL

TABLE OF CONTENTS
PART 6.1 PIPELINE PRESSURE CONTROL 6-1
6.1.1 Need for Pressure Control 6-1
6.1.2 Pressure Reducing Valves 6-1
6.1.3 Grade Break at Tank 6-4
PART 6.2 SURGE CONTROL 6-6
6.2.1 Pressure Tank as Surge Chamber 6-6
6.2.2 Minimize Frequency of Pump Cycle 6-7
Flow Control Valve 6-9
Flow Controlled Pressure Switch 6-9
Pump Cycle Timer 6-9
6.2.3 Install Air Valves 6-11
6.2.4 Use Slow Closing Valves 6-11
6.2.5 Control Flow Rate at Float Valve 6-11
6.2.6 Operation Plan 6-11

FIGURES
Figure 6.1 Typical Pressure Reducing Valve Installation 6-2
Figure 6.2 Pressure Valve Parts and Design Charts 6-3
Figure 6.3 Float Valve Box 6-5
Figure 6.4 Operation of a Surge Chamber 6-7
Figure 6.5 Remote Multi Tank Installation 6-8
Figure 6.6 Flow Rate Controller Valves 6-10
Figure 6.7 Flow Controlled Pressure Switch 6-11

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MT Stockwater Pipeline Manual Pressure and Surge Control
NE Supplement

CHAPTER 6
PRESSURE AND SURGE CONTROL
6.1 PIPELINE PRESSURE CONTROL

6.1.1 Need for Pressure Control

There are frequent circumstances in long pipelines where the operating

pressures at hydrants are too high. Due to the limitations of hydrant
and float valve mechanisms, maximum pressure at a hydrant and/or float
valve should be limited to not more than 80 psi. Where pressure exceeds
80 psi, it should be reduced before flow is turned into the valve.

The cost of high-pressure pipe can sometimes be reduced by installing a

pressure reducing station in the pipeline. This allows using a pipe
with lower pressure rating. The cost savings must always be weighed
against potential operation and maintenance problems which are
frequently a result of installing a pressure-reducing valve.

There are two ways to reduce pressure in a segment of pipeline: (1)

Install a pressure-reducing valve and (2) Install a tank with a float
valve and a gravity pipeline extension.

* Pressure reducing valves or tank/float valves should be used as a last

resort. They are mechanical devices that can and do sometimes go wrong.
In many cases, there is no other way to maintain pressures below 80 psi,
so a pressure-reducing device must be installed.

Examples in Chapter 9 show how to perform hydraulic calculations where

pressure reduction is required.

6.1.2 Pressure Reducing Valves

Figure 6.1 illustrates a typical pressure reducing valve installation.

* NEBRASKA NOTE

Tank/float valves refer to an in-line float valve box as shown on

page 6-5.

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Figure 6.1
TYPICAL PRESSURE REDUCING VALVE INSTALLATION

Pressure reducing valve size should be selected based on manufacturer's

recommendations. Too small a valve will create very high velocities in
the valve and cause rapid valve failure. Too large a valve will cause
poor pressure regulation.

If the valve installation as shown in Figure 6.1 is at a high point

in the pipeline, an air release valve or combination valve may also
be required, as described in Chapter 7.

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Figure 6.2 illustrates construction of a typical pressure-reducing valve

and a manufacturer design chart.

Figure 6.2
PRESSURE VALVE PARTS AND DESIGN CHARTS

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* The valve illustrated in Figure 6.2 has a built-in strainer. Pressure

reducing valves will not operate properly if debris gets into the
mechanism. Many pressure-reducing valves have the small built in
screens shown. Sediment and debris are enough of a problem in some
pipelines that the screen soon becomes clogged. A more elaborate filter
system may be required. If so, the types of filters used in home filter
systems or trickle irrigation systems can be used.

Manufacturer's charts show the maximum capacity for each size of valve
based on design velocity. The flow rates in stockwater pipelines are
usually so low that maximum flow rate is usually not a problem.

There is a pressure reducing valve pressure loss called "Fall-off" that

must be considered in design. When no flow is passing through the
valve, there is zero fall-off. When maximum rated flow is passing
through the valve, there is up to 20 psi pressure fall-off. So if the
pressure reducer is set at 75 psi at no flow, the static hydraulic grade
line would be at (75 x 2.31) = 173 feet above valve elevation. If the
valve were to operate at design flow, hydraulic grade line would start
at the valve at [(75 - 20) x 2.31] = 127 feet above valve elevation.

Figure 6.2 illustrates typical manufacturer information concerning valve

fall-off values.

6.1.3 Grade Break at Tank

Starting a gravity pipeline at a tank is one positive way of controlling

pressure in a segment of pipeline. If the float valve hangs up, the
tank simply overflows. Both static and dynamic hydraulic grade line
starts at the water surface in the tank. Only a usually insignificant
pipeline entrance loss is experienced under design flow.

Figure 6.3 illustrates one type of tank/float valve installation. This

is a small tank with a float valve strictly used for pressure
regulation. A stock tank can be used in the same way. A strainer
should always be added at the intake.

* NEBRASKA NOTE

When a pressure-reducing valve is used in a design, the plans should

indicate, as a minimum, the pressure differential and the allowable
"fall-off" pressure for the design flow rate. This information will be
sufficient to allow the installer to obtain the appropriate valve from
the supplier.

Use of more than one pressure-reducing valve in a pipeline may result in

oscillating pressures if misused or improperly installed. When more
than one pressure-reducing valve is used, the design shall be approved
by an engineer with appropriate job approval authority.

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MT Stockwater Pipeline Manual Pressure and Surge Control

Figure 6.3
FLOAT VALVE BOX

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MT Stockwater Pipeline Manual Pressure and Surge Control

6.2 SURGE CONTROL

Surge (water hammer) can be a serious problem in long stockwater

pipelines. Consider what happens when a two mile long pipeline is
suddenly shut off. The entire mass of water in the pipe is moving in
the direction of flow. When the water is suddenly shut off,
considerable force is required to stop the momentum of the large water
mass.

The actual pressure build up depends on the total volume of water in the
pipe, velocity at which the water is moving, and how fast the water is
stopped. Pressures can be much greater than operating pressure, and can
even be greater than static pressure in the pipeline.

In low head, low pressure pipelines, surge is usually not a problem. It

is almost always a factor that must be considered in long, high pressure
pipelines.

A frequent surge problem is encountered on pumped systems. When the

pump shuts off, the water starts to reverse in the line. A check valve
closes, setting up a pressure wave and cyclic pressure surges. If the
pump system contains an automatic pressure switch, the pump can rapidly
cycle on and off causing damage to the pump, pipeline, and valves.

Another frequent cause of surges is rapidly turning off a hydrant.

Frost free hydrants can be shut off very rapidly by slamming down the
handle. This is sure to cause surges in the pipeline. Float valves
will also be turned rapidly on or off if something causes the water in
the tank to slosh around.

Ways in which surge can be controlled include:

6.2.1 Pressure Tank as Surge Chamber

For automatic pressure systems, a properly maintained pressure tank will

act as a surge chamber. The air bubble in the pressure tank acts as a
cushion for water reversing in the pipeline. Figure 6.4 illustrates how
a surge chamber works.

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Figure 6.4
OPERATION OF A SURGE CHAMBER

Sometimes when pressure at the pump is very high, a normal pressure tank
cannot be used. In that case, it may be necessary to install a high
pressure rated diaphragm-type pressure tank, or specially designed surge
chamber. These are expensive but may be needed in high pressure
systems.

It is sometimes proposed that a homemade surge chamber be installed.

This is a piece of pipe capped at one end and an air valve installed in
the outer end. The chamber is filled with compressed air after the
system is pressurized with water.

Homemade surge chambers are not recommended. Experience and studies

have shown that this type of chamber soon waterlogs and becomes
completely ineffective.

6.2.2 Minimize Frequency of Pump Cycles

Minimize the frequency of turning the pump on or off. This will reduce
the number of surges that the pipeline and system will have to endure.
This can be accomplished by increasing pressure tank storage. Figure
6.5 illustrates a remote multiple tank setup for increased storage. If
remote tanks are used, one high pressure tank should also be installed
as close to the pumps as possible. This tank will provide a little
storage and, most importantly, will act as a surge chamber.

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MT Stockwater Pipeline Manual Pressure and Surge Control

Figure 6.5
REMOTE MULTI-TANK INSTALLATION

Remote tanks can generate problems of their own. When the remote tank
is far out on the pipeline, hydraulic conditions can be such that,
during initial pump flow, friction loss in the pipe will cause pressure
to buildup to cut out pressure and turn the pump off before the remote
tanks have filled to design pressure. As pressure in the system
equalizes, the pump will again start. A rapid cycling can be set up
which can be very destructive to pump and pipeline.

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MT Stockwater Pipeline Manual Pressure and Surge Control

Three possible solutions to this problem are:

(1) Flow Control Valve

If this problem is encountered, one solution is to install an

adjustable flow rate control valve in the pipeline near the pump.
With this valve, flow rate is adjusted downward until rapid cycling
is stopped. Figure 6.6 illustrates this type of installation and
two types of flow rate control valves. These valves are expensive.

(2) Flow Controlled Pressure Switch

There is a pressure regulator/pressure switch combination valve

which works so that once the pump comes on, it will not shut off
until all flow in the system has stopped. This guarantees that the
pump will not cycle except between flow events. Figure 6.7
illustrates this type of valve. There are two models with different
flow rate ratings. At least two pump manufacturer's supply this
type of valve as an accessory.

If either of the above two valves are used make sure that the pressure
rating of the pipe between the pump and the valve is high enough to
withstand the maximum pressure the pump is capable of generating. This
will require a review of the pump curve. With these types of valves the
pressure between pump and valve will reach the maximum that the pump is
able to generate.

(3) Pump Cycle Timer

Another possible solution is to install short period timers in

conjunction with the pressure switch. The timer is set in a manner
that will force minimum pump on or off cycle times. It will be
especially important to have adequate pressure tank storage and
pressure relief valves installed if this alternative is selected.

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Figure 6.6
FLOW RATE CONTROLLER VALVES

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MT Stockwater Pipeline Manual Pressure and Surge Control

Figure 6.7
FLOW CONTROLLED PRESSURE SWITCH

6.2.3 Install Air Valves

Remnants of air in the pipeline can set up conditions that promote

surges. Air valves or vents should be installed in the pipeline to
remove air under pressure. See Chapter 7 for more details on air
removal.

6.2.4 Use Slow Closing Valves

Install slow closing gate valves instead of frost free hydrants. Gate
valves must be installed in access risers so that they are below frost
line.

6.2.5 Control Flow Rate at Float Valve

Control the maximum flow rate through a float valve by installing an

orifice or flow control valve.

6.2.6 Operation Plan

Provide an operation plan to the operator cautioning him to close valves

slowly and otherwise operate the system in a manner which will minimize
surges in the system.

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