SURGE OR WATER HAMMER
Since the early 1950’s, Parco Model DE Surge Control System has set the standards for a low head loss pump discharge and control valve system to protect pumps and increase their lives. Parco Model DE Surge Control Systems have been designed, manufactured and used on many installations in USA and worldwide.
WHAT IS SURGE?
Surge, or pressure surge, results when the velocity of a column of water is changed.
The change in velocity may be an increase or decrease. The magnitude of the pressure surge or downsurge is directly proportional to the rate of change in the fluid velocity.
WHAT CAUSES SURGE?
– The starting and stopping of pumps employing simple check valves are a common cause of pressure surges.
– Rapid valve operation is often blamed for creating surges. But it is not the valve so much that is to blame, but the speed of the valve’s operation.
As a “rule of thumb” in a rigid pipe such as ductile iron for example, for eachfoot per second in fluid velocity, a potential for 50 to 60 psi surge could result.
The velocity change must occur within one surge period, meaning the time required for one round trip of the surge wave traversing a single pipe.
For example, in a pipe 4000 feet long, it will require a surge valve traveling at 4000 fps twp seconds to traverse the length of the pipe and back. In this example, each foot per second change in velocity in two seconds or less, a surge of about 60 psi would result.
This surge, which is an increase in pressure, must be added to the existing static pressure to get total pressure.
The speed at which surge wave travels in water is governed by pipe diameter, wall thickness, and modulus of the pipe material.
1- Close valves slowly, and longer pipelines require slower valves closure times. Fire hydrants are famous for causing surge because they are usually closed rapidly. Such problems can be avoided if the hydrant’s closing speed is gradually reduced with an occasional pause as the hydrant approaches the seated / closed position.
2- Use proper pump control valves with a system that has built-in safety controls and features that protect the pumps from surge and assure dependable operation of the pumps, even when unattended.
THIS IS WHAT PARCO MODEL DE SURGE CONTROL SYSTEM PROVIDES.
WHAT IS CAVITATION
Cavitation denotes the vapor cavities formed just downstream of the point of maximum velocity, which is the point of minimum pressure, which is at the smallest opening at the valve seat or throttling element.
Cavitation is often associated with pumps.
However, cavitation is an important factor in valve application, as it affects valve life.
HOW DOES CAVITATION OCCUR
Cavitation occurs when a fluid is passing thru a valve that is required to throttle or modulate, as energy of the fluid is being converted from pressure to velocity.
The increase in velocity means a decrease in pressure.
When the decreased pressure reaches near the normal water vapor pressure (of about 0.25 psi), cavitation “bubbles” will form.
POSSIBLE DAMAGES BY CAVITATION
The vapor cavities or “bubbles” formed will collapse when they enter the higher pressure region where the fluid once again slows down to the pipeline velocity, and velocity energy is converted back to pressure energy.
The collapsing bubbles (pin size each) or cavities can generate pressure of many thousands of psi.
Thus, should the collapsing bubbles be in contact with the pipe wall or valve body, damage will result.
When thousands of bubbles burst in a concentrated area, it can fatigue the metal surface and produce a “pit”.
Continuous exposure to cavitation can destroy a valve body and/or the pump impeller or volute.
Example: Pressure Reducing Valves with more than 50% pressure reduction in one valve (200 psi to 100 psi versus 110 psi to 10 psi).
PARCO MODEL DE SURGE CONTROL SYSTEMS
PURPOSE AND FUNCTION:
Parco Model DE Surge Control System has many built-in safety controls and features for an ASSURED dependable operation of pumps, specially if they are unattended on 24 hours basis.
STANDARD CONTROLS/ FEATURES AND BENEFITS:
Parco Model DE Surge Control System provides, as standard, the following features/controls:
- Pump Protection in the event of loss of prime; If pump fails to prime or due to mechanical fault fails to reach normal pressure, Parco valve will remain closed and controls will cause safety shutdown cycle.
- Prevention of pump operation against a closed valve, which means that if the Parco valve fails to open for any reason, there are controls that would prohibit the pump from operating.
- Prevention of “run-out” of the pump if the line breaks downstream
- Complete Manual operation for testing or pump isolation
- Parco’s exclusive hand control valve allows manual hydraulic operation to open, close or throttle the surge control valve regardless of whether or not the solenoid valves are energized.
- Pump restart prevention after a power failure unless and until the valve is closed.
ADDITIONAL OPTIONAL CONTROLS/ FEATURES AND BENEFITS:
- Rapid closure in the even of a power failure (DEF). It closes quickly through an independently adjustable speed control on power failure
- Level Control of reservoir and wells (DEL)
- Downstream pressure control (DEP)
- Backpressure control (DEBP). This is an adjustable back pressure control feature which modulates the valve based on operator adjustable set points.
- Differential pressure control
And many other controls or combinations.
Parco designs its own hydraulic actuation system called Depend-A-Power, which includes accumulators, and is connected to the control cabinet of the DE surge control system thru hydraulic piping.
PARCO CABLE-TORQUE OPERATOR
Parco Model DE Surge Control System uses Parco’s unique cable-torque operator which was designed specially for better surge control service, as this mechanism must withstand shock loads during power failure.
All of the control components for the pump discharge surge valve are located in one cabinet, in all NEMA ratings. It can be operator mounted, meaning over the cable-torque operator, or wall mounted or supplied with floor stands.
Depending on the application, it is possible to have one control cabinet controls two separate valves which must be operated in a particular sequence.
SURGE CONTROL: Parco has many years of experience in sizing and selecting surge control systems. For use with our systems, we select only those valves that do have low head loss, such as butterfly valves, cone valves and certain ball valves. These type of valves assist in the control of surge due to their low head loss features as compared to globe valves, angle valves, gate valves and check valves.
(See the 4 graphs at end of Parco catalog).
HOW SURGE IS CONTROLLED:
1- The common check valve and swing check valve open when the impact of the pumped water contacts the disc of the valve. Water pressure holds the valve open until such time as when the operator shuts down the pump, at which time the return back-flow of water from the system strikes the valve disc form the opposite side and bangs it to its seat. On such installations, the pump is shut down, after which the valve is closed by the back-flow of water, and HAMMER AND SURGE are induced.
2- Now, let us compare this with the operation of the Parco Surge control system where the valve does NOT open UNTIL THE PUMP COMES UP TO SPEED AND PRESSURE REACHES THE SET POINT. Opening time is adjustable simply and easily. When the system is filled and shut down time arrives, the operator does NOT SHUT DOWN the pump, but instead simply de-energizes the pilot solenoid .
The valve begins its closing cycle, which is timed by our controls, with the pump continuing to function. WHEN THE VALVE IS APPROXIMATELY 80% TO 90% CLOSED, depending on the type of pump control valve used, the switch on the valve shuts down the pump, and the valve continues slowly to close to the tightly closed position, bringing the water flow to rest with minimum disturbing elements involved. Thus, the valve closure is accomplished quietly, completely and positively without bangs, hammer or surge.
3- In the case of Butterfly Valves used in the Parco system, there is a rapid closing speed for 80% to 85% of the closure, and then very low speed for the remaining 15 to 20% of the closure.
This is how and why the Parco Model DE Surge Control System controls normal pump start-up surges, normal pump shut-down surges, closes valve rapidly and automatically on power failure to prevent reverse flow, can be manually opened partially or totally or be closed.
SERVICES & REPUTATION:
Parco takes pride in the fact that it has proven its ability to design and build surge control equipment to match the needs of consulting engineers’ and customers’ hydraulic and mechanical designs. We do assist Consulting Engineers and owners/end users in sizing and selecting surge control systems and valves types that are best suited and is most economical for a user’s application.
For example, in the Central Water Pumping Station for the city of Dammam, Saudi Arabia, we have assisted Metcalf & Eddy, a prominent and large USA consulting engineering firm with worldwide offices and projects, to size and select the appropriate Parco Model DE Surge Control Systems for Dammam Central Pumping Station. These are as follows:
1979 – Parco #7965 sold to Weir Pumps Limited, Glasgow Scotland. The order consisted of :
Two (2) 300mm, and
one (1) 400mm Parco Model DEFP Surge Control Systems with Fast Close Feature (closes quickly through an independently adjustable speed control on power failure) and adjustable back pressure control feature which modulates the valve based on operator adjustable setpoints.
There was no accumulator.
This system uses tandem cylinders, one power cylinder, one slave cylinder.
Operating medium is supplied by system pressure.
1992 – Parco #9212 sold to Somiral Supplies SA, consisting of:
One (1) 400mm Parco Model DEFPX (same as above)
1992 – Parco #9231 sold to Flow Treat, Inc. Rutherford, NJ consisting of:
One (1) 300mm, and one (1) 400mm Parco Model DEFPX (same as above)
PARCO MODEL DEF
SURGE CONTROL SYSTEM – OPERATION
(For more controls details, refer to Wiring Diagram B-05029)
- With valve in the closed position, start pump motor.
- When pump develops minimum set required pressure (field adjustable) against Parco surge control valve, Parco valve on pump discharge will open at a field adjustable speed and pump now discharges to the system.
- When Parco valve is partially open, both valve open and valve close lights will be lit. When the valve is fully open, the valve close light will go off.
- Open contacts of manual or pilot device.
- The motor starting equipment is still energized and pump is still running.
- Parco valve will close at a field adjustable speed.
- When Parco valve reaches the closed position, the controls will de-energize pump motor and pump will stop.
SAFETY SHUTDOWN FEATURE
PARCO VALVE FAILS TO OPEN
If mechanical difficulty prevents Parco valve from opening, high pump discharge pressure against the valve would initiate the controls to start safety shutdown cycle.
PARCO VALVE FAILS TO OPEN FULLY IN SPECIFIED TIME
If the Parco valve fails to open fully in the time set period, controls will initiate a safety shutdown cycle.
LIMIT SWITCH FAILS TO STOP PUMP
If lightning or other high electrical energy fuses contacts of limit switch or if improper adjustment of limit switch causes pump to continue running with Parco valve closed, controls will initiate safety shutdown cycle.
PUMP FAILS TO REACH NORMAL PRESSURE
If pump fails to prime or due to mechanical fault fails to reach normal pressure, Parco valve will remain closed and controls will cause safety shutdown cycle.
In the event of a power failure, controls will close the Parco valve rapidly to prevent reverse flow.
STARTING PUMP WITH VALVE OPEN
If the pump motor is stopped or for some reason the valve fails to close, it will be impossible to restart the motor until the valve is in the fully closed position.