Pumps and Systems, April 2009
In today's growing industrialized market, the need for reciprocating pumps is increasing daily. As technology drives modernization, the demand placed on the pumping systems is continually rising.
Early piping systems were typically designed for less stringent circumstances, which have often created surges within the newer operating conditions. These surges, known as water hammer or pulsation amplification, are phenomena generated within the design of the system or the operation of the pump.
Using Newton's Third Law of Motion, "For every action there is an equal and opposite reaction," we are able to determine that pulsation amplification is a reaction generated through five different sources.
- Unsteady friction
- Fluid-structure interaction
- Cavitation
- Blockage and leakage
- Elasticity of pipe
(Source: Bergant and Tijsseling 2001)
Unsteady friction within the system is described as fluid traveling through a pipe. Fluid moves faster at the center point of the pipe, and the fluid will drag or be slowed closer to the pipe wall. As this fluid drag interacts with the fluid at the center point, a turbulent action causing amplification occurs within the system.
Fluid structure interaction is the physical change of a product that would cause a physical change in a product from point A to point B. Cavitation is a physical change within a system where a gaseous state can occur normally due to a pressure drop within a system.
Blockage and leakage is normal within most operating systems and usually results from a safety valve opening or closing or a reciprocating pump fluid end valve's normal operation.
Pipe elasticity is a concern when using a non-rigid system such as PVC. PVC can actually expand slightly and temporarily hold more fluid than it is designed to contain. This expansion of fluid capacity is then forced into a smaller diameter due to joints or connections which increases the velocity of the fluid through that connection. This action causes amplification waves within the system.
The regeneration of these amplification sources has justified the need for better fluid controls. Primarily, piping fatigue, metering malfunctions or system vibrations are the early signs that pulsation amplification exists within the system. Ultimately, the amplification can be diminished within the design, but due to the cyclical nature of reciprocating pumps, it cannot be eliminated. Suction stabilizers and discharge dampeners can be inserted within the system at strategic points to help mitigate pressure spikes.
Discharge Dampeners
Discharge dampeners were developed to dampen the effects of the amplification of a pumping mechanism. In the late 1950s, discharge dampeners were developed to attenuate pressure spikes on single acting production pumping oil wells. These wells were creating excessive spikes that could break piping flow lines and manifold collection points. The need to control these pressure spikes was rising at an alarming rate. Different methods considered at the time actually decreased overall well production, which pushed the need for more control and facilitated the development of new-age pulsation dampeners.
Pulsation dampeners were initially placed within systems as a bandage that only covered the problem. Even today, that is all they do: protect downstream instrumentation and piping from excessive surges. As pressures within different industries increased, the dampeners became less efficient than originally anticipated. The focus turned to a solution instead of just masking the problem.
Suction Stabilizers
Suction stabilization is a means of stabilizing the fluid and its erratic motion before the fluid can be amplified within the reciprocating pump. This attenuation of the fluid is known in the industry as stabilizing, so the term suction stabilizer became common. Suction instability is normally generated through the blockage and leakage of the pump valves. As the suction valves close, a blockage is created within the system, which causes the fluid to stop, reverse its motion and generate a shock wave. The pump valves open and leakage is generated, forcing the fluid to start in motion again. To return this fluid in motion requires a certain amount of inertia, called acceleration head calculated (HAc).
HAc= LVNC / Kg
Where:
L= Length of Pipe
V= Velocity
N= RPM
C= Constant (.115 Duplex, .066 Triplex, .040 Quintuplex)
K= Compressibility Factor (Water 1.4)
g= Gravity (32.2)
(Source: API 14E)
Stabilizers absorb the acceleration head effect via the charged cartridge, providing increased fluid flow throughout the pump and system.
The use of both suction stabilizers and discharge dampeners within the same operating system will allow for the least turbulent action to be generated. The control on the suction side of the pump absorbs the acceleration head, prompting the fluid to stay in constant motion. This attenuation is generated through the use of a nitrogen-filled cartridge or bladder. The nitrogen charged bladder is compressed by the pressure surge and decompressed upon demand generated by the suction line pressure decrease. The fluid will enter the suction cavity and be available to move through the suction valves with the least amount of turbulence. The less turbulence entering through the suction valves results in a minimum pressure spike that can be amplified within the pressure cavity of the pump.
Once the fluid reaches the pump valving, monitor the fluid's velocity. Neglect in maintaining proper fluid velocity through pump valves in relation to the speed of plunger travel (crank rotation) will result in a mid-stroke implosion. As the fluid flows through the valves, it must fill the pressure cavity of the pump while keeping a solid mass of fluid on the face of the plunger. If the fluid separates from the face of the plunger, a pressure drop occurs. In the event this pressure drop falls below the vapor pressure of the fluid, then tiny air bubbles will form on the plunger service. If the pressure does not rise above the fluid vapor pressure before the forward stroke of the pump, the bubbles will be atomized into thousands more bubbles. These bubbles are known as cavitation and under pressure can cause a severe, damaging fatigue to a pump's valves and downstream instrumentation.
Non-stabilized piping systems allow the acceleration head effect to continually send water surges through the pump and piping system.
Not all reciprocating pumps need pulsation equipment but almost all pumps generate some pulsation due to the cyclical nature of their design. As the crankshaft makes a full rotation of each plunger and starts its next rotation, the plunger stops for a fraction of a second. This stationary moment in the rotation allows the fluid to completely flood the pressure cavity. In the case where complete flooding of the fluid end is not achieved, voids are formed.
The need for pulsation equipment is normally determined by the pressure spikes generated from the operating system. If these spikes are deemed detrimental to the operation, some form of attenuation is suggested to eliminate costly downtime and unnecessary maintenance. Normally, these two issues are the determining factors when deciding to install equipment on existing systems.
The four different styles of stabilizer are cylindrical appendage style, in-line, no maintenance and mushroom/bladder type.
Conclusion
If piping systems are designed to eliminate an excessive amount of acceleration head effect, some HAc remains due to blockage and leakage (suction valves opening and closing) within the system. Suction stabilization will absorb the motion of the fluid within the compressible charged bladder as the suction valve closes. When the suction valve opens, the compressed gas will react to the pressure difference, decompressing the fluid simultaneously with the suction valve opening. This action within the stabilizer will keep the fluid in motion-attenuating the unsteady flow.
If the suction valves have a sufficient amount of fluid under them they will seat softer, disallowing the seats to be driven deeper into the fluid-end decks. The decreased amount of turbulence within the pressure chamber will decrease the amount of vibration, which is transmitted through the plunger into the power end of the pump. This decrease of vibration will increase life of crankshaft bearings, other power end and fluid end components, rewarding the user with a reduction in general maintenance operations.
In general, watching the flow characteristics of the fluid before it enters the pump can increase pump performance and decrease major unscheduled maintenance.