Surge arrestors are designed to control rapid velocity changes which may cause potentially dangerous pressure excursions.
One method of accomplishing this is by using a vessel charged with inert gas and connected to the pipeline carrying the liquid. This vessel has the capability to convert the kinetic energy of the moving liquid into stored potential energy when a liquid overpressure occurs. When a pump stops, the vessel gas expands and “pumps” needed fluid into the line to prevent the formation of vacuum and column separation. This hydro-pneumatic feature gives the surge arrestor its ability to control the rate of energy change.
Major Applications: Surge Arrestors for Pump Shutdown and Start-up
By storing and releasing energy into the pipeline system at the appropriate time, the surge arrestor is able to balance energy changes and control the pressure change.
Function During Pump Shutdown
Upon pump shutdown the pressure in the pipeline downstream of the pump check valve begins to decrease. This change in pressure is sensed through the connecting flange causes the gas to expand, forcing the stored liquid out of the surge arrestor into the pipeline like a pneumatic pump until the pressure stops decreasing. At this time, the surge arrestor will be nearly emptied of liquid and the pipeline pressure will be lower than the system static. This minimum pressure exists only as a transient and the system immediately begins adjusting itself by returning flow to the pump area. The returning liquid flows into the surge arrestor with increased resistance induced by the differential friction device. The differential friction device has a low pressure drop flowing out of the surge arrestor and a high pressure drop flowing in.
The control of down surge plus the high level of energy dissipation by the differential friction device keeps the return pressure rise below the predetermined limits required for the pipeline system.
Function During Pump Start-up
On pump start-up, the downstream column resists movement and causes the pressure to rise at the pump to shut off pressure head or even higher. When vertical turbine pumps without foot valves start, liquid rises in the pump column at high velocity without resistance and slams against a check valve held shut by static liquid in the pipeline, producing severe shock.
With a surge arrestor installed, flow from the pump is received within the surge arrestor compressing the stored gas. Pump energy thus stored is released in a controlled fashion to establish steady state flow.
A Surge Arrestor for Flow Stoppage
The Greer hydro-pneumatic surge arrestor may also be designed to prevent excessive transients in liquid pipelines caused by a sudden reduction of flow. By storing excess energy and releasing it under controlled conditions, the surge arrestor is able to balance the energy changes and prevent pressure from exceeding the design limits.
Function During Flow Stoppage
As a valve is closed in a line connected to the surge arrestor by the flange, the pressure in the line adjacent to the valve begins to rise. The gas in the surge arrestor is compressed as liquid enters the surge arrestor. Inflow continues until the pressure rises to the design point. At this time, the flow ceases and the surge arrestor will be filled with liquid and a small volume of compressed gas. Pressure will now be freater than the system static pressure and compressed gas will “pump” liquid back into the system. The differential friction device controls the outward flow, stabilizes the system and minimizes pressure oscillations.
Other Applications: Valve Closure
Valve closure applications are usually associated with the user rather than the supplier as the latter generally does not require quick closing valves. However, users frequently require them for such things as fire and deluge systems, fuel transfer and loading, line switching, et. Surges thus generated are easily controlled with a surge arrestor.
Pressure Regulating Valves
Control valves may cause rapid flow and pressure variation. A surge arrestor installed upstream of such a valve will stabilize its action.
Special Models Available for Hugh Turbidity Waters
Surge arrestors are adaptable to high turbidity fluids such as storm drainage, sewage and slurries. The design of such surge arrestors takes into consideration the prevention of solid accumulation and provides flushing features if experience indicates it is necessary. Some sewage systems generate gas and so gas vent valve may be necessary.
Mechanical Features: Low Maintenance
The Greer surge arrestors have no moving parts except and elastomer bladder. Therefore, no maintenance is required except to replenish or check the gas charge. The pressure vessel shell is constructed of carbon steel and fabricated in accordance with ASME Boiler and Pressure Vessel Code, Section VIII, Division I. All wetted metal surfaces are protected with special coating to prevent corrosion. This eliminates the associate maintenance and vessel replacement corrosion would cause.
Installation Requirements: Pump Shutdown
The surge arrestor is to be installed on the downstream side of the pump check valve as close to the valve as possible. The surge arrestor may connect to the discharge manifold at any point if the distance from the check valve is less than 25 feet/ Where it is not possible to locate the surge arrestor this close to the pump check valve, it may be necessary to increase the connection pipe in diameter and/or the surge arrestor size.
The surge arrestor is to be installed immediately downstream of the pump check valve with the connecting line to the surge arrestor at an angle to the pump header forming not more that 45 degrees with the flow looking at the pump or with a generous tee.
Pump Discharge Check Valves
Performance of the surge arrestor is predicated on each pump being equipped with a quick closing check valve or the pump control valve acting as a check valve. This is necessary because the volume of liquid stored within a surge arrestor is just sufficient to limit down surge to the specified amount. If the check valve is of the slow closing design, liquid from the surge arrestor will be lost through the check valve and the pump. Check valves considered quick closing by Greer are the tilting or slant-disc type, spring-loaded-center-guided-silent type, spring-loaded-center-hinge type. If a simple flapper check valve is used, it must be heavily externally spring-loaded