Minimum Pump Recycle Control Arrangement
Minimum Pump Recycle Control Arrangement
(OP)
We are providing automatic minimum flow recycle for pumps. The simple arrangement is to measure the total discharge flow and use it to control the recycle flow through a control valve located on recycle line.
The reason is that the control valve doen't know how much flow to recycle. An improved version could be adding one more flow measurement so that the actual flow through recycle line can be interpreted from total pump and the flow to process. This way, the controller can be properly tuned.
I want some experience with the above arrangements and which one is better.
Thanks for your responses.
The reason is that the control valve doen't know how much flow to recycle. An improved version could be adding one more flow measurement so that the actual flow through recycle line can be interpreted from total pump and the flow to process. This way, the controller can be properly tuned.
I want some experience with the above arrangements and which one is better.
Thanks for your responses.





RE: Minimum Pump Recycle Control Arrangement
An easier solution, for which a flow diverting Yarway valve works nicely, is to base the recycle flow on a maximum discharge pressure with a recycle control valve set to open proportionally when discharge pressures go higher than the discharge pressure set point. But this solution does depend on your control philosophy and if flow measurement is needed to control your system rather than controlling off of the max discharge head setting.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Minimum Pump Recycle Control Arrangement
Doug
RE: Minimum Pump Recycle Control Arrangement
I agree with djack77494. Yo don´t care about the recicled flowrate. The limitation for the internal of the pump is established by the total flow and this design value is the one which never has to get lower. Install just one flowmeter to measure the total flow. Tuning a flow controller is really easy, it can be done in minutes and the tuning parameters (gain and integral time) don´t depend on knowing the flow through the valve. You just need a stable and quick responde of the PID but you don´t need a fine tuning for this service.
RE: Minimum Pump Recycle Control Arrangement
You can't say he doesn't care about minimum flow. There are many cases where minimum flow may be the only control parameter, as the OP suggests.
One loop feedback on total-flow control might be appropriate where the pump is constant speed, however if the pump is variable speed, diesel driven for example, and minimum flow must be assured at whatever flowrate determined by some downstream condition, multiple or variable flow set point or flowrates, such would be exactly the case and a one-measurement point flow feedback is not appropriate.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Minimum Pump Recycle Control Arrangement
RE: Minimum Pump Recycle Control Arrangement
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Minimum Pump Recycle Control Arrangement
Doug
RE: Minimum Pump Recycle Control Arrangement
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Minimum Pump Recycle Control Arrangement
The total flow through the pump is the only measure required for minimum flow control, however, since the net flow (total-recycle) is often required for other control purposes, the total flow scheme usually results in an extra flow measurement. Still this is the best scheme unless you use something like the Yarway flow diverting valve cited by BigInch above.
Incidently these self contained minimum flow valves are not really pressure activated, rather they work more like a piston rotometer. When there is a forward flow the piston moves forward and reduces the minimum flow in the side branch. When there is not enough forward flow the piston slides back causing more flow to be diverted to the side branch (minimum flow). I think they work fine for relatively clean service and some plants where I work use them for practically all minimum flow applications with great success.
best wishes,
Sean
RE: Minimum Pump Recycle Control Arrangement
RE: Minimum Pump Recycle Control Arrangement
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Minimum Pump Recycle Control Arrangement
----------------------------------
Sometimes I only open my mouth to swap feet...
RE: Minimum Pump Recycle Control Arrangement
It is surprizing how many VFDs get attached to systems where the only real need was a properly sized pump or at times just a tank or a valve.
A VFD can only be justified by a need for widely variable flowrates, or in situations where replacing a pump to obtain the "proper" flowrate is impossible for one reason or another AND the cost of power running at the pump's BEP flowrate is too high. Otherwise, VFDs are just another often expensive headache and in some cases where flows are variable the system might run much better just by adding a tank. Many people seem to think they can be the "snake oil cure all" for any mismatched pump.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Minimum Pump Recycle Control Arrangement
There are many ways to ensure this is kept, either by an orifice control to ensure a constent minimum flow, but this also entails constant energy waste into product recirculation, variable speed drive by smart monitoring which can slow motors down, and commonly pressure releif valve on the recirc line, so when process calls for the liquid at a higher flow, and in turn lower pressure, all energy is put into transferring liquid as opposed to sending it back to the supply tank again.
Hope this is of use.
RE: Minimum Pump Recycle Control Arrangement
IMO 10% might be an absolute minimum for starting up purposes ONLY, which should be run up to 20% ASAP and maybe kept there for a very limited time, until system interlocks go off and the system is ready, then right on up to op speeds.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Minimum Pump Recycle Control Arrangement
A centrif pump running at 10% of BEP will be sufficient to eliminte heat generation due to the disk speed of the Impeller, however, MTBPM will be effected as there are hydraulic thrusts to consider and take into account (running at 60% will have reduced effect on these internal thrusts).
On a standard chemical process pump application, many processes are used for batch transfer, and so recirculation is for only a short period of time, if we are looking for long term recirculation, then VSD will be by far the best option merely from an energy cost point of view, as well of course as the MTBPM issues associated with recirculation
RE: Minimum Pump Recycle Control Arrangement
Yes, that's why I said 10% absolute minimum.
For "long term" recirculation for mixing a batch or some process flow reason, as opposed to recycling to provide pump minimum flow, VSD is no solution. I contend a "long term" flowrate needed for process reasons, that is 20% or so of a batch transfer rate which was made the pump BEP flow, should probably have a separate pump sized for the process rate and you should probably not be using the same pump running at 20% of its BEP. True a VSD would save power, if it could also provide enough head to do the recirc, but at the cost of possible increased breakdown time and increased maintenance. It may be cost effective in some cases to do that, but what's the target?... proper operations or proper bean counting?
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com