Minimum Pump Recycle Control Arrangement

[ixchawla]

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.

 

[BigInch]

A flow measurement in the recycle line is definately easier to implement mathematically and is usually much more accurate, but does cost some addtional hardware. You might be able to do it with a simple orifice plate in the recycle line used to roughly measure flow and send that signal to a control valve in the recycle line.

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

 

[djack77494]

For systems that justify the capital expense involved, I would employ a single flow control loop that measures the total pump discharge flowrate and modulates a valve in the recycle line when that flowrate drops near/below the desired minimum flow. You don't care what the recycle flowrate is, so why worry about measuring it? Like BigInch, I believe a Yarway Automatic Recycle Control (YARC)valve is an alternative. It avoids the cost of measurements, transmitters, actuators, etc. Some folks like them and some don't, so you might just want to inquire at your or your client's facility.
Doug

 

[hanon]

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.

 

[BigInch]

Guys,

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

 

[dcasto]

BigInch, what everyone is saying is that we don't care what the flowrate through the minimum flow line is, we care what the flow through the pump is. If the minimum flow for the pump is 100 GPM and the process demand is 200 GPM, the minimum flow valve is shut. Likewise if the system demand for the above is 75 gpm, who cares if 25 gpm is going through the control valve loop, we only care that 100 gpm is going through the pump.

 

[BigInch]

Yes, I know and I'm wondering why, since the OP states "We are providing automatic minimum flow recycle for pumps". Apparently he is worried about it.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[djack77494]

I would classify a single control loop consisting of a measurement of the total flow (through the pump) and a minimum flow line with control valve as providing "automatic flow recycle for the pumps". The objective is met by this arrangement.
Doug

 

[BigInch]

True, but how do you make it work, if the pump shall be variable flow? Readjustment and probably retunning of the control settings would be needed every time the pump speed was changed.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[sshep]

Ixchawla,

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

 

[dcasto]

On one of our 1000 HP pumps, the minimum flow changes through the RPM range of 25% by 30 HP. The minimum flow drops fromm 450 GPM to 350 GPM across that same range of speed change. So we would waste somewhere between 0 and 35 HP. We thought about putting a varible setpoint for minimum flow in the PLC and though about again and we ended up not doing it. The pump is in pipeline service and the time we spend at low flow was next limited, but worth having a varible speed.

 

[BigInch]

3% savings when on minimum flow for what? maybe less than 5% of the time.......Humm.... DEFINATELY not worth the trouble.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[ScottyUK]

Return on investment for a 1000 HP VSD sounds like it would be a long time in that application, especially if the drive is an medium voltage type which are expensive items in anyone's language. Also the electrical reliability is lower compared to not having a drive simply through the increased component count, although granted there might be some improvement in mechanical life by avoiding the stress of DOL or RV starting. VSDs are great devices but the cost / benefit / ROI needs to be weighed up for each application.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[BigInch]

Great advice Scotty.

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

 

[pumpking]

As a pump manufacturer (cdrpumps.co.uk) we see too many times pumps that fail prematurely due to pumps running at too low a flow. As a rule of thumb the minimum flow should be 10% of the flow rate at the best efficiency point on selected Impeller curve (the performance curves should detail the efficiency points).

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.

 

[BigInch]

API says 60% of BEP flow is the minimum continuous flow.

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

 

[pumpking]

API says 60% of BEP for a number of reasons, mainly due to reliability point of view, the API spec is a heavy duty spec for the petrochemical Industry, involving high pressures, high temperatures etc.

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

 

[BigInch]

pumpking,

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