December 08, 2007

Never Ignore Pump Bypasses?

Many of you, especially who are handling projects engineering, may be surprised when & why should I consider bypass for pumps. What is the utility for considering bypasses? Why not only relief valves are sufficient for safeguard against high pressure in the line? The list may be little longer than my expectation…. Here are few things to remember….

Here is the list of few important considerations for Bypasses.
  1. For all Positive displacement type pumps (PD pumps) full size bypasses are always essential in the form of safety relief valves back to the suction line or suction tank.

  2. All the equipments or instruments before the bypasses in item-1 above should be designed up to 1.5 times of the set value of relief valve.

  3. To prevent excessive temperature rise at prolonged low flow operation compared to design value, a bypass is necessary for

    a. High discharge head pumps due to higher rate of temperature rise at low flow and,

    b. Liquids operating near saturation temperature. Normally, BFW (boiler feed water) pumps are equipped with automatic regulators for bypass flow due to availability of both these factors in one pump.

    c. Usually 10% of normal flow is sufficient in such cases as bypass flow.

  4. To avoid unstable operation below certain point in case of some pumps it is necessary to provide bypass. This condition arises out due to internal re-circulation which may result in vortex cavitations, low frequency pulsation and, vibrations. This normally occurs if Ns (Suction specific speed) is very high at 10000 or more.

  5. Automatic bypass is necessary for pumps, which are controlling level or temperature instead of flow. In such cases variations are usually large & prolonged and it may have to run at shut-off also due to process disturbances or during start-up / shutdown.

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