Chemical & Process Technology

"Is there a rule of thumb used in determining the flow required through the recycle line on the pump discharge in case of minimum flow problem in pump? If yes, what percentage of the pump flow is considered to flow through the recycle line."

Strainght forward answer...


Percentage can ranged from 10% to 50% (or even 60%) of PUMP FLOW may be considered during design phase. I am recommending 30%-40%...However, this figure shall always be checked & confirmed with actual selected pump when they are manufactured.

There are at least four (4) main factors possibily determining pump MINIMUM RECYCLE flow. There are :

a) Fluid temperature rise
b) Minimum stable flow
c) Internal recirculation
d) Thrust capacity

Fluid temperature rise
As fluid enters pump suction chamber, pump impeller will spin the fluid and energy from shaft / impeller is transferred into fluid. However, there are mechanical losses within the pump in the energy transferring process. The mechanical losses will be transformed into acoustic energy (e.g. noise) and thermal energy (e.g. fluid temperature rise). Acoustic energy may transfer to fluid, pump casing and piping and it is further emitted to atmosphere as noise. Similarly thermal energy will also transfer to fluid, pump casing and piping. However, pumping process is a rather “fast” action and in general there is “insufficient time” for heat transfer from fluid to pump casing and piping. Thus, majority of the thermal energy will stay in the fluid and eventually cause fluid temperature rise (and/or liquid flashing). Reduce fluid passing pump results less heat “carrier” and higher temperature rises and high potential of liquid flashing. Thus a minimum flow can be established from fluid temperature rise. A process engineer may needs to establish minimum flow of a pump from temperature rise perspective. The principle is rather simple where


Above has considered acoustic energy is zero.

Fluid temperature rise,

Where
h = pump head
g = gravity acceleration
e = pump shaft efficiency
Q = pump flow
Cp = fluid specific heat

Minimum system stable flow
Sometime a pumping system may shows two stable flows at certain pump head. As a result, it ”hunts“ or ”shuttles“ between these two flows and potentially damage the pump and other equipment within the pumping system. For example, gas trapped in the discharge line pocket. Trapped gas will reduce liquid flow path and increase line pressure drop. Pump head increase push trapped gas towards downward piping. As trapped gas move into the downward piping, liquid flow path increases and reduce the pump head. Trapped gas will form smaller bubble due to fluid turbulence and it will rise in the downward piping and finally back to the high pocket again. Similar cycle occurs again and pump oscillate in two flows. A process engineer shall always piping system to avoid pump operate in the oscillation region.

Internal recirculation
As flow rate decreases in the pump chamber, flow reversal will occur at the pump suction and discharge vanes. Recirculation vortex will form at both ends and potentially damage pump. Thermal heat gain within vortex will further increase pump fluid temperature and potential flashing occurs.

Axial thrust load
Axial thrust in a pump increases rapidly as flows are reduced and head increased. A minimum flow needs to be maintained so the thrust developed by the pump does not impair bearing life.

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