Properly mitigating system shaking & water hammer

Sept. 7, 2016

Surge analysis software helps protect against hydraulic transient.

Process control engineers and operators face many challenges to keep pumping systems running efficiently and reliably. For users of positive displacement (PD) or PD-style pumps, the issues of addressing damaging pulsations caused by the stroke of the pump are all too common and unavoidable. These pulsations will often violently rattle or shake piping, causing noise, premature wear and potentially a complete break in the pipe itself or damage to valuable components in the flow path downstream of the pump. Traditionally, the solution has been to install a pulsation dampener on the discharge side of the pump.

A pulsation dampener is a hydropneumatic device that, when attached close to the discharge side of the pump, can mitigate these pulsations and steady the flow. With the dampener, vibration becomes nearly nonexistent and laminar flow replaces the previous fluid turbulence. Consistent flow suitable for measuring, metering, mixing and many other applications is now readily available. Several designs are available for pulsation dampeners and one of the most common is a cylinder tank shell with an elastomeric bladder on the inside.

The bladder is charged on the non-wetted side with either nitrogen gas for chemical and industrial applications or compressed air for water and neutral solutions. The bladder, constructed of chemically compliant elastomer material, functions only as a containment vessel for the gas. The compression of the gas inside the bladder absorbs the pressure energy of the pulses. Since the number of pulses is dependent on the speed of the pumps and the overall system, the pulses can often have an extremely high frequency rate. Variance will be present and will depend on the specifics of each unique application. The bladder will be nominally pre-charged with the approximate gas at a level of 75 percent of the system’s minimum operating pressure. This will keep fluid in the dampener the majority of the time except when all fluid is released from the dampener while the pump operates during the low-pressure cycle. When the pump is running, the high-frequency gas compression and expulsion not only maintain steady continuous flow but also keep the mean pressure within the system.

This technology is well-established in the industry as an accepted solution for the reduction of pulsations when PD pumps are used. Typically the pulsation dampener is sized to the PD pump and not the complete system. What is often overlooked when sizing a pulsation dampener are the additional components of back-pressure control valves, safety relief valves, pump suction stabilizers, active valves and check valves (See Figure 1 below). Why is consideration of additional components necessary when the primary concern is one of sizing the pulsation dampener? The answer is that while a pulsation dampener can absorb the positive energy of the pump’s pulse, it is not always able to address a potentially far greater source of damage to the pump and piping system caused by a hydraulic transient event.

Figure 1. System setup for an oil refinery, showing the placement of a pulsation dampener, relief valve and back-pressure valve

Water hammer causes

A hydraulic transient, known more commonly as "water hammer" or "surge event," is generated whenever a change in the flow velocity of a pumping system occurs. During a change in flow velocity, a dramatic rise or drop in pressure will occur with potentially catastrophic results. This flow velocity change can be caused by a pump stop ("trip") from a power loss, emergency stop or even unanticipated intervention through mechanical, pneumatic or personnel means.

If the transient occurs during a pump stop, a negative pressure wave will rapidly travel downstream of the pump at speeds as high as 4,000 feet per second depending on the fluid type, viscosity, pipe elevation, pipe roughness, bends, etc. When the wave goes out, an immediate drop in pressure can result in negative pressures that can break the pipe by collapsing the pipe walls. During the pump stop, a signal is often sent to close a check valve downstream, and the closing of this valve will then come into contact with the negative pressure wave. This results in a dramatic rise in pressure with the high-pressure wave now travelling back to the pump at a speed equally high to that of the negative wave. A check valve is usually placed on the discharge, but it may not close quickly enough to prevent the high-pressure wave from passing by and striking the pump, causing damage. Once the check valve closes fully, the pressure wave will travel back and forth between it and the closed downstream valve, resulting in water hammer and the loud sound of pipes shaking and rattling. The pressure wave will eventually disperse, primarily because the pipe walls will absorb its energy, which causes premature wear. The negative wave generation necessitates that energy be given to the system to keep the pressure from dropping below 0 psig. Unfortunately, the stored fluid (available energy) inside the pulsation dampener can be sized too small to have a significant effect on the pressure drop.

When the pump is operating and a valve is suddenly closed downstream, a high-pressure wave is generated that will travel back toward the dampener and pump. The close rate of a check valve may not be sufficient to prevent all or some of the high-pressure energy from reaching and damaging the pump. Once the check valve is fully closed, the water hammer effect continues unabated until its energy is absorbed by the system’s pipe walls and devices in the flow path. Because this is a high-pressure event, should the installed pulsation dampener be able to assist, if not outright mitigate, the effect of the high-pressure wave? After all, it is designed to accept energy by compression of the gas inside the bladder.

Surge analysis software solutions

The solution is to look at the piping system — particularly the components mentioned earlier — comprehensively. Surge analysis software can generate a simulation model of the piping system in which different conditions can be tested and evaluated independent of the actual system. Prior to the availability of the software, this type of analysis was a time-intensive and laborious process for engineers. Now the analysis will quickly perform the calculations and processes necessary to quantify the nature and extent of the transient in addition to determining what are the preferred equipment options to solve the problem.

Many calculations and sizing such as liquid to gas volume change can be field verified through the incorporation of a differential pressure transmitter on the pulsation dampener during normal PD pump operation. The measurement of discharge pressure downstream of the PD pump can be determined with and without a pulsation dampener. A pressure relief valve installed on the discharge side of the pump that is designed to open from the transient and system isolation will be sized from the software, and a back-pressure control valve can be sized to sustain a preset pressure. Once the analysis is completed, often the result will indicate surge vessel use.

Upon first inspection, a surge vessel appears to be equivalent in design and functionality to a pulsation dampener except that it is much physically larger in size. The design is one of a tank with an elastomeric bladder inside that contains the correct gas suitable for the application. However, pulsation dampeners are designed to absorb very small amounts of energy from the high-frequency rate of pulses from the pump. Thus, they can be sized compactly. Surge vessels must give or accept energy in large amounts during a single transient event to quickly prevent major damage to the system’s pump, piping and connections.

When installed close to the discharge side of the pump and under pump trip conditions, the surge vessel must be sized large enough to immediately deliver sufficient fluid to the piping system and prevent negative pressures from forming that could lead to column separation and pipe collapse. For sudden valve closures downstream, the surge vessel is placed as close as possible to the inlet side of the valve. The sudden closing of the valve creates the high-pressure wave energy that can be absorbed by compression of the gas inside the surge vessel’s bladder.

Through the use of surge analysis software, a PD pump system can be protected against pulsation damage from the pump’s stroke but also from potentially costly and catastrophic transient events. The software’s model will cover numerous potential outcomes and the results can be field verified at the installation site with transient monitoring equipment.

Figure 2. A transient monitoring system captures the water hammer effect in a system.

Transient monitoring equipment is specifically designed to read system pressure changes at 500 times per second and record data at 100 times per set because of predetermined boundaries established by the user that are unique to the application. This data is stored until reviewed for content analysis (See Figure 2 above). This type of analysis allows for confirmation of the model’s expectations of performance with quantifiable and documentable results from the job site. While the sizing process for a pulsation dampener is not as extensive as that required for a surge equipment solution, the end result is a pump/piping system designed and confirmed for efficient and reliable performance.

Frank Knowles Smith III leads the Blacoh Surge Control team as the executive vice president. He is a trailblazer in fluid dynamics with three decades of academic, design and application experience. Smith’s specialties include pump station/pipeline design and computer modeling, piping components, instrumentation, and electrical control panel design.

Steve Mungari is the business development manager for Blacoh Surge Control. He was previously the western regional sales manager for Burkert Contromatic Inc., the USA division of Burkert Fluid Control Systems, which specializes in the field of fluidic measurement and control technologies to the life science, gas handling, hygienic process and water treatment industries. 

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