Surging pump 17-12

The pump is surging 17-12

  • Random surging is caused by an air pocket getting loose in the suction piping
    • Dissolved gases in the system
    • The pump suction is below atmospheric pressure (vacuum) and air is coming into the packing, Stop it by converting to a mechanical seal.
    • The fluid is vortexing at the pump suction. Use a vortex breaker
    • An air pocket at the suction caused by a reducer that has been installed upside down
  • Constant surging is usually a result of a low NPSHA
    • Internal recirculation is heating the liquid to its vapor point
    • A stuffing box suction recirculation line is being used with fluid at or near its vapor point
    • The well is not keeping up with the pump.

Some thoughts on pump surging

  • If you have a pump with a flat curve (low specific speed), a small variation in pressure (head) can cause a significant change in flow rate.
    • If the suction side of the pump is under vacuum conditions, a liquid at elevated temperature, close to its vapor point is always a problem
    • Is there anywhere in the system that vapor can accumulate and cause random surging as it eventually finds its way to the pump?
  • You could be seeing a very high frequency “water hammer,” except instead of one or two “hammers” that is usually associated with a water hammer, it is appearing as a high frequency vibration or pulsation.
    • At any restriction in the pipe, a valve, or a change of pipe area, a certain portion of any pressure wave is reflected, while the balance continues down the pipe.
    • When a restriction is located at a point where the pressures are reflected in “tune,” or at a particular harmonic being generated by the pump, vibrations can increase in amplitude.
    • It is much more commonly known in reciprocating pumps and compressors, Centrifugal pump pulsation is less commonly known, apparently because it is much less of a problem for most users.
  • One method used to avoid flow surging problems is proper location of the discharge throttle valve.
    • When a centrifugal pump is operated at a very low flow rate, recirculation occurs within the impeller, and it surges at the natural frequency of the system. As the control valve is moved away from the pump, there is a decrease in the frequency and an increase in the amplitude of the pressure waves
    • The energy imparted to the system by the pump is similar to the strumming of a guitar string. The frequency is a function of the length of the string, and is analogous to the distance from the pump to its control valve. The greater the distance between the valve and pump, the lower the frequency of this oscillation and the greater the magnitude of the pressure pulsations.
    • Placement of the valve close to the pump discharge flange minimizes the amplitude, and thus the effects of the flow oscillations. The mass of the oscillating fluid is reduced in volume, and the turbulent flow through the valve destroys the frequency of the excitation force.
    • If the throttle valve is remotely located, flow surging will be of low frequency and high amplitude since the fluid mass is large. This situation must be avoided, as it may induce violent mechanical vibrations in the piping system.
    • The preferred throttle valve location will always be close to the pump discharge flange in order to minimize the potential and the effects of flow surging.
    • Flow surging problems can also be resolved by installing a bypass line. Bypassing a portion of the pump capacity back to suction maintains pump operation closer to its design flow where the amplitude of the hydraulic excitation forces is small. The bypass line also protects the pump from overheating and damage if system flow rate is reduced below minimum flow.

System Stiffness

  • The stiffness of the pumping system affects both the frequency and amplitude of pump surging. A “soft” system can result in low frequency high amplitude surging that is detrimental to pump performance and life.
  • A “soft” system can be caused by a number of factors:
    • There can be air or gas entrained in the pumping fluid, air can be trapped in an elbow, or a system can have open tanks or an accumulator.
    • A closed loop boiler with a remotely located control valve that takes only a minimal pressure drop at low flow rates, or a system in which there were flexible lines attached to the suction and discharge of the pump, would be typical of a soft system.
    • The following recommendations should be applied to minimize the effects of soft systems:
    • Place the control valve at or within five feet of the discharge of the pump.
    • Use flexible discharge lines only downstream of the control valve and use a minimum of 10 pipe diameters of straight piping into the pump suction.
    • Size pump control valves so that 5-8% of the differential pump head is taken across the valve.
    • Install a low flow bypass system if operation below minimum flow is anticipated
    • Eliminate entrained air and gasses, or anticipate a higher minimum flow limit if they cannot be avoided.
    • Avoid air traps in pumping systems.
    • Consult your pump supplier if operation is anticipated below 40% of the BEP for any sustained period and the NPSH is less than 30 feet.
    • In hydraulically soft processes where a capacity control valve cannot be located near the pump, and maximum turndown is needed, a discharge orifice is often recommended. This isolates the pump from potential system resonances, and permits stable operation at flow rates down to 15-20% of BEP (the pump’s best efficiency point). The orifice is normally sized to drop 5% of the pump head at BEP.

Posted

  • On February 13, 2018