Positive Displacement Pump Protection 1

[ChemStell]

I am looking at an application where we are using fixed speed gear pumps (Viking) to pump fuel additives to tanker loading bays. The existing system seems completely over-engineered and I would like to design something simple and effective, in terms of pump protection. There is no internal relief valve on the pump. There is an external relief valve mounted directly on the discharge with its outlet piped to the suction side.

The pump is connected to the additive storage tank.
The application will start the pump when there is demand for the additive at the additive injector units. When the demand stops, the pump continues to run for a few minutes (existing procedure, I believe there is a minimum run-time to limit the amount of stop-starts) and the end of the line is dead-headed, there will need to be some sort of pump protection, as I do not believe the relief valve should be used as an operational flow device, but only a safety device.

My first question is: Is a re-circulation line back to the tank the best way of provding the pump protection?

My second question is: How do I correctly size the line? If it is too large, it will be pumping too much liquid back to the tank, instead of to the additive injectors. If it is too small, when the line is dead-headed, it will not allow enough flow back to the tank, and probably lift the relief valve anyway.

Anybody who has come across the sort of application before?

 

[psafety]

I'd start by correcting the pumping "procedure" that seems the best place to start.

 

[dcasto]

Ok, instead of calling it a relief valve, its a maximum differential pressure valve, who cares if it looks like a PSV. If you don't like it, look up a Cashco valve. Your car has one just like it on the fuel rail, runs all the time.

as psafety suggests, tell us why there is a time limit or limit on stop starts. Unless you are pumping some HUGE amounts of additive, I can't imagine such a thing (I guess you could have undersized the system and it is prone to overheating).

 

[JHanson717]

Instead of adding more valves, maybe consider using a N.O. 3-Way "diverter" valve. Instead of dead heading when it shuts off it would divert it back to tank or around the pump. I use the Whitey SS ones with pneumatic actuators on my PU resin systems to keep the fluid moving...
//jh

 

[unclesyd]

No matter what type diverting system you use you will still have to a positive relief, rupture disc etc, at the pump discharge in case of diverter failure

 

[MJCronin]

Call Viking to see if you can add a properly sized relief device to the pump casing. This might involve tapping a hole.

Q1: Yup...it buys you time. Make sure that the tank nozzle is robust and that you are not making "tank foam" because the nozzle is too high up on the shell

Q2: Make it the same size as the pump discharge...Pipe is cheap.

My opinion only

Please let us know of your final decision.

Consider posting a system "PID"

Regards

-MJC

 

[TD2K]

Most companies I've done work for won't allow a pressure safety valve to be used as the pressure control device. Normally on something like this I would use a backpressure regulator to maintain pump discharge and recycle at no flow and a PSV to protect against blocked discharge.

One valve I have seen those companies accept for continous control and overpressure protection in a single valve is a Fluflo valve.

 

[tickle]

Another valve is a constant flow valve from Yarway

Often over pressure devices inside pumps can get blocked and do not operate when required. You would need to consider this in the HAZOP process.

Ebsray pumps have a relief valve on the pump casing that is basically a PSV. I would consider that as being adequate for over pressure protection.

 

[ChemStell]

Thank you for all your contributions.

psafety / dcasto: I am still struggling to get data about the plant operation that appears to have been lost over time. One explanation I have been given for the pump not stopping immediately once the signal from the additive piston injectors is taken away, is to act as a grace timer, so as to prevent spurious stops.

tickle: The Yarway seems to be a good application, however only used for centrifugal pumps.

If I haven't explained the process above clearly: The existing process has a pressure safety / relief / max diff. press valve on the pump discharge, which is piped into the suction. Downstream of the pump is a recycle line, fitted with a pressure relief & regulating control valve. (i.e. it only lifts once the spring set pressure is overcome). This pressure is only likely to be overcome when the end of line is dead-headed (i.e. demand stops) and the flow is diverted towards the recycle. In addition there is a bypass line around this regulating valve with an globe valve (shut under normal operation). And around this, is another bypass, with an orifice plate! (This I assume, is to allow some flow through the pump, while the pump is running. I'm speculating that when running, a piston injector temporarily dead-heads the end of line due to its operation). The more I look at it, the more it appears that this system has been designed for every operating scenario.

Your thoughts / input will be appreciated. Does this seem like a good design or can this be simplified?

 

[ScottyUK]

One possible reason for limiting the number of starts is to protect the motor. The rotor absorbs a lot of energy during starting which heats it up, and it needs a period of running to allow the heat to dissipate by the action of the internal fans moving air through the stator and the external fan blowing air over the casing. Depending on how the big the pump is it might be limited to three or four equi-spaced starts per hour with a period running in between to get the heat out of the machine.


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

If we learn from our mistakes I'm getting a great education!

 

[HEC]

why are you using a PD gear pump, could you get the required head and flow using a pneumatic diaphragm pump. These can be set to provide regulated number of strokes to provide accurate dosing. As they are pneumatic/solenoid driven, there is no problem with minimum run time.

An alternate solution to the minimum run time is to use a soft starter. The starter will require additional cooling to get around the heating problem.

There are many ways to remove the fur from a cat!


Mark Hutton

 

[tickle]

I have installed a Yarway on a rotary vane pump. We got the valve and pump from the same supplier. Who did the sizings.