recirculation valve

[garfio]

This is kind of a general question but based on a real situation. In a fuel loadout pumping system, the loading centrifugal pumps have a backpressure valve to control the pressure in the system, and at the same time, assure a minimum flow in the pumps.

A second set of centrifugal pumps (smaller), inject other fuel to the same system. For this second set of pumps, it has been specified relief valves in the recirculation line. there is a flow control valve downstream the second set of pumps to assure the injection is done in the correct proportion. The relief valve would be there only to satisfy the minimum flow.

I wonder if backpressure valves would be a better choice for the second set of pmps for the following reasons:
- Recirculation control would be more gradual. Relief valve now tend to pop open as safety valves do, posibly affecting the control of the right proportion. Blow down can also affect the system.
- A backpressure valve would be easier to regulate in the field.

Might be that the relief valve is cheaper, but it doesn't look like the right choice. What do you think?

 

[Montemayor]

If the scope of what you want to accomplish is to ensure that the minimum flow allowable is at least maintained in a centrifugal pump that is susceptible to dead-heading or drastic flow reductions, than you should be applying something like Yarway’s automatic recirculation control (ARC) valve.

So-called “relief” valves (much as are used in positive-displacement gear pumps) are meant for safety purposes, never for process control. To rely on this kind of relief device to control flow is to misapply it and never really accomplish what the scope is intended to do. The Yarway valve comes as close to protecting a dynamic machine such as a centrifugal pump as there is available in a simple, unitized device. Go to Tyco’s website and download the literature pertaining to the model that applies to your application:

http://www.tycoflowcontrol-na.com/brand_literature_results.asp?Selection=Yarway

 

[gerhardl]

Difficult to generalize, but yes, it is always better to go for a smoothest possibel regulating sequence, and that is actually what you always try to obtain: not the fastest, but the smothest and safest, best adapted to your process and total equipment installed.

That again will, at a given process, mean that you have to check all variabels and possible points to influence or regulate the process, not limiting yourself to the pipeline and valves.

In addition to valves (types, placement, CV, sequence)this could begeneral for the process you describe: pumps (types, capacity, soft start or stepless regulating), consumption and requirements for flow regulation, allowable pressure and mixture, viscosity, temperature, pipeline size and layout,pressure peaks ('waterhammer'), startup and closedown time restrictions, measure instrumentation and measurement requirements (if any), safety rules and restrictions ..etc, etc....

 

[JJPellin]

Are you certain that the smaller pumps are centrifugal? If they are positive displacement a relief valve is necessary to protect against overpressure if they were operated with a blocked outlet. If they are centrifugal, a standard PSV would be a poor choice for low flow protection. As already noted, a recirculation valve would be a better choice. It might be possible to use a full-flow type relief valve that would open gradually rather than pop open.

But just because these pumps are running in parallel with other, larger centrifugal pumps, do not assume that they are also centrifugal.


Johnny Pellin

 

[Scipio]

Something else to bear in mind, those PSV's may be there because even a centrifugal pump can develop enough pressure at shutoff to overpressure a piping system. That could be the case here too, in which case the PSV may be required to satisfy piping code.

 

[garfio]

Thanks for the input.

I can tell for sure that the pumps in the second set are also centrifugals. Also, the main group of pumps even at shutoff, won't be raising the pressure above the design pressure of the system.

As an additional data (I just didn't want to complicate the case), there is a third set of pumps (this time positive displacement for sure) injecting additives to the system. This set do have relief valves. What I don't like from these pumps is that they also have a flow control valve downstream for the same reason (correct proportioning of the additive). That looks to me as a very poor design, because again, the relief valve will be opening each time the control valve closes to adjust the flow rate. Been small pumps, I think that a better designed system would have the controller acting directly over the driver of the pumps (controlling speed), and leaving the relief valve to do just what it is design to do, protect from overpressure.

 

[Scipio]

If we're just talking minimum flow protection the I think Montemayor has the most practical idea with ARC valve. As pretty much everyone agrees, PSV's are not built for this kind of thing. First, like has been mentioned, they're snap-acting, and the pumps will wind up hunting back and forth as the back pressure in the system starts jumping up and down as the PSV lifts and reseats. Second, unless you have a very steep pump curve, there's no way the PSV will reliably lift where you need minimum flow protection, all it's sensing is the pressure. ARC's operate based entirely on flow going through them - I've heard they can be a little twitchy if you have varying viscosity going through them, but for fuel loading that shouldn't be a problem.

Regarding that third set of pumps, back pressure control doesn't do a thing in terms of varying the flow out of a positive displacement pump (I'm assuming that's how someone's tried setting yours up, if the PSV lifts as the valve throttles). The only two ways you can control those is by recirculation or, as you suggest, by speed control (sometimes stroke adjustment is an option on certain reciprocating designs).