LOGOS Learning · 7
Pump minimum flow: a pump running on the left of its curve
Pump minimum flow is the lowest flow a centrifugal pump can run at without overheating or unstable operation; a recirculation line keeps it there by adding a second path: Q_pump = Q_process + Q_recirc. An orifice plate in that line burns the spare head and sets how much returns to the tank.
The pump is too big for the process line and runs far to the left of its curve. Turn on the recirculation with an orifice plate and watch the pump return to a healthy flow.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. Why avoid running the pump far left on its curve?
2. With the fixed-plate recirculation on, the process gets…
3. Drawback of fixed-plate recirculation:
Why this happens
Arrows compare with recirculation off at the same level.
Continuity at the discharge node
The pump now sees two paths: the process and the recirculation. Pump flow is the sum of both.
Why the pump must not run far left
Away from BEP the pump runs with internal recirculation, heats up, vibrates and wears seals and bearings. The LOGOS calculator warns when flow drops below 50 % of the largest flow among the curve points and recommends recirculation.
The plate burns the spare energy
At the node the head is the same for both paths. In the recirculation, whatever is not spent climbing to the crest and on pipe friction must be burned by the orifice plate: that permanent loss sets the bore diameter.
The price of protection
At higher flow the pump delivers less head and the process gets slightly less. And all recirculated water is wasted energy. That is why processes with variable demand use controlled recirculation (minimum-flow valve) instead of a fixed plate.
Teaching pump with an illustrative curve.
Formulas in plain text
- Continuity at the discharge node
Q_pump = Q_process + Q_recirc- Q_pump = flow through the pump (m3/h) · Q_process = flow delivered to the process (m3/h) · Q_recirc = flow returned to the source tank (m3/h)
- Low-flow warning (LOGOS calculator rule)
Q_pump >= 0.5 * Q_max,points- Q_max,points = largest flow among the pump curve points entered (m3/h)
- Head balance on the recirculation path
H_node = dz + hf_recirc + dP_plate- H_node = head at the discharge node (m) · dz = rise from the node to the line crest (m) · hf_recirc = friction loss in the recirculation pipe (m) · dP_plate = permanent pressure loss of the orifice plate, as head (m)
Frequently asked questions
What is the minimum flow of a centrifugal pump?
It is the lowest continuous flow the manufacturer allows, below which the pump suffers internal recirculation, heating, vibration and seal and bearing wear. It is often quoted somewhere around 25 to 50 % of the best efficiency point flow, but the pump datasheet governs.
What happens if a pump runs below minimum flow?
Far left of BEP the liquid recirculates inside the impeller, the temperature rises and radial loads grow. The result is vibration, noise and short life of seals and bearings.
How is a minimum flow recirculation line sized?
Pick the recirculated flow that brings the pump to a safe point, then size the orifice plate so its permanent loss equals the head left over at the node after the elevation and pipe friction of the return line.
Fixed orifice or minimum flow valve?
A fixed orifice always recirculates, which wastes energy. Processes with variable demand usually use a controlled minimum-flow (bypass) valve that opens only when the process flow drops.