Check valve in the suction line of centrifugal pump

[Pumpinterest]

Anybody has ever seen a check valve installed in the suction line of a centrifugal pump? I am now in a design project, we have a buffer vessel for a centrifugal pump and the bypass line for the buffer vessel, in normal condition the centrifugal pump the fluid through the buffer vessel, but in abnormal condition the buffer vessel will has to switch to the bypass line. To prevent the fluid reverse to the buffer vessel we desgin a check valve in the outlet of the vessel. I want to know, if I consider the pressure drop of the check valve to ensure the NPSHA at the pump suction point above the NPSHR, is there any problem to the centrifugal pump.
Can anybody answer me,if that I will appreciate your help very much.

 

[Montemayor]

Pumpinterest:

I'm not being fascious, but in the interest of strictly answering your question the way it is posed: Yes, it is common to install a check valve ("foot valve") in most sump and vertical pumps - especially those that have their suction nozzle totally submerged in the pumped fluid. This check, of course serves to keep the pump primed and ready to start pumping on subsequent starts when instrumented with a stop-start capacity control.

Your english and writing skills need polishing; your sentence isn't very well structured and hard to understand. However, if you're recycling back to the pump suction and don't want the recycle to go into the tank source, you're going to have trouble with your NPSH. I doubt if you'll get an NPSHr amenable to your check valve, but if you can predict (with accuracy) that the check won't jeopardize your NPSHa, then go right ahead. But, like I said, I doubt it.

 

[Pumpinterest]

Montemayor,
Thanks for your suggestion.
I have ever seen the foot valve you mentioned, it is used for maintenance. is it right?
I know that the pressure drop across the check valve maybe very small, but the pressure to open it maybe larger. In the low flow rate condition, the check valve maybe shock.
You know I am in a design project, so I should have enough excuse to persuade other people to change the this design. Could you help me?

 

[Montemayor]

Pumpinterest:

The foot valve is operational all the time. It has nothing to do with maintenance. It serves the needed purpose of keeping a vertical pump's suction always primed -i.e., always flooded or filled with liquid. Otherwise, as I'm sure you'll understand, the pump couldn't start up pumping liquid. You must prime the pump - just like all pumps have to be primed. Except that in the case of the vertical pump, the liquid falls out of the pump and the connected piping the moment the pump stops - that is, if it doesn't have a check ("foot") valve in the suction line that is kept in the source liquid.

I think we have to have a correct and accurate description of what it is you are trying to accomplish from a process point of view. This is what is difficult to envision or to try to engineer. If you describe your operation (& your goals) in detail, perhaps we can all contribute a simple and acceptable answer to your needs.

Await your reply.

 

[rzrbk]

If the pressure drop of the check valve is considered in the NPSH calculation, then the pump should function correctly. However, this opens up several design and reliability problems like you mentioned above, a swinging disk or flapper causing suction pressure to swing, check valve won't be fully open on startup or low flows, possiblity of check valve sticking, etc.

Not sure what flows are going where on your system, but if there is any way an additional check valve on the discharge of the pump would work, (maybe after the branch)this would be preferred.

If a stop is required on the suction of the pump, consider an actuated valve which would trip closed on low/reverse flow, or when the bypass is actuated. This will be more expensive than a check, but be more reliable. Depending on the consequenses of reverse flow to the vessel, this may be justified.

 

[rmw]

The answer to your question is simple and has already been given by rzrbk and Montemayer. I only repeat it for clarification. If you have figured the pressure loss across the check valve (wherever it is in the pump suction piping,) in your NPSHr calculations, and you have adequate NPSHa to meet the requirements of the NPSHr for the full flow range of the pump, you should have no problem.

Chattering of a check valve or foot valve (shock as you put it) at low flows is a different matter, and you have to avoid low flow conditions by taking care of that in your design.

A foot valve as mentioned by Montemayer above is a type of check valve that is used in the suction line of a pump designed to "lift" water from a source at a lower level than the pump, such as a sump or tank or drum lower than the pump. It is attached at the very initial point that the liquid enters the line from the sump or lower vessel. Your application as I am able to understand it is not that.

rzrbk, as I read his original post, it might appear that there is a check valve at the vessel outlet too to prevent backflow when the pump is pumping in the bypass loop. But I am unsure of that.

Pumpinterest, while your english is good, it would be helpful that you should give us a detailed explaination of the process so that we can understand it better.

rmw

 

[Pumpinterest]

I am sorry for my poor english.
after rmw's explaination, I understand what the check valve Montemayor mentioned is use for.

What rmw's understanding of my question is very well. the check valve is applied to prevent the backflow from the bypass loop beacuse of the higher pressure in the bypass line.
The fluid media is LNG. the buffer vessel is the BOG(boil of gas) recondenser.Beacause there is a level control valve in the upstream of the recondenser, so the pressure in recondenser is lower than the upstream pressure.the upstream pressure is normal about 15Bar, and the recondenser pressure is normal about 8Bar. when the level is high high in recondenser, the bypass valve will be fully opened to maintain flow to centrifugal pump and then close the inlet and outlet of the recondenser after a time delay. During the switching period, the pressure in the bypass line(equal to upstream pressure 15Bar) will be higher than the pressure in recondenser beacause of less pressure drop in bypass valve, the back flow to recondenser could be happened. to prevent this back flow, a check valve is installed at the outlet of the recondenser. that means in normal operation a check valve is in the suction of the centrifugal pump, so I want to know if there is any problem on the pump starting and running.

 

[rmw]

You don't have to apologize to me for your english. I might have to apologize for mine.

Your last post helps clarify your situation. So the answer already given in previous threads still applies. and that is this: If you have accounted for the pressure loss of this check valve along with all the other pressure losses in the suction piping to the pump, and you have more NPSHa than NPSHr (and some safety margin) then your pump will start and run OK.

Chattering, or sudden closing during changing from the flow through the recondenser to the by pass line might occur at times, but it should not be a problem for the pump as long as it gets adequate flow from one source or the other at all times.

While the foot valve mentioned by Montemayor is commonly used in most sump applications, it is not the only type of check valve used in pumps where it is desired that the fluid not flow back to the source when that source is located below the pump, so those pumps that use flapper or disc type check valves start and pump through those check valves day in and day out throughout their running life without ill effects.

It is up to you as the design engineer to make sure you include enough pressure loss in your suction piping calculations for this check valve as well as all the other resistances in the suction piping and add some safety margin to make sure that the NPSHr requirements of your pump are met.

rmw