4 replies to this topic

[jprocess]

Dear All,
What is a foot valve?Is it a kind of check valve?
What is its role regarding to NPSHa for suction lift pump application?
Thanks in advance.
Warm Regards.

[Art Montemayor]

Jprocess:

I can understand your confusion over the term. Many times, non-USA engineers are forced to confront what have been "popular" or common nicknames given to process equipment out in the actual, original field applications by operators and not by the original design engineers. This has happened a lot in what we call the "oil patch", here in the USA. This industry and expertise was pioneered and took root here and, consequently, develped a lot of field-oriented nicknames for equipment. It is not until a USA engineer -like myself - is forced to work out of the USA and to have to operate in a foreign environment - complete with a 100% immersion in the local language - that one realizes that it is very, very difficult to literally translate these terms into the local language. The translations come out sounding ridiculous! Another term that gives a logical problem is "reboiler". Liquid is BOILED - not REboiled. The term makes no logical sense. If it made sense we would have to say that our mothers reboiled water every day to makes us tea. Of course, the term came about because there was a need to differentiate between it and a Boiler - a completely different device. However, the popular term still sounds illogical or is a bad description.

This also applies (in a way) to the term "foot valve". A foot valve is nothing more than a swing check valve operating in the vertical position at the the liquid entry to the vertical suction line of a sump pump or another type of pump that has a negative suction lift due to gravity - i.e., the pump is usually located above the liquid level of the suction vessel or container. If there is any logic in this description it is that the check valve is located at the "foot" of the vertical pipe run. But it has to be understood that we are talking about a check valve - not another type of valve. The function of a foot valve is to contain the liquid inventory within the pump casing and the suction line in the event the pump is shut down or stopped. This action preserves the liquid priming of the pump and allows for a successful, instant liquid pumping start up during the next, subsequent pump start-up. This is especially useful when the pump is started from an isolated location and there is no one around to manually prime the pump with liquid prior to start up.

Unfortunately, as most good ideas go, the foot valve results many times causing more problems than it resolves - not due to its design, but rather to the practical fact that when one has a negative suction lift on a pump, it is often because the application involves something similar to an open sump. An open liquid containment system below grade is, by nature, a great collector of strange and foreign contaminants - such as waste paper, lunch bags, welding rods, excess Teflow tape, and a lot of other "garbage" items that prevail at plant sites around the world. And when this happens, all this garbage gets into and cancels the sealing action of the foot valve. All that is left to the operator is to install a screen or grating around the entry to the suction line, but this results with the screening device getting clogged up eventually with time. So, the result is that the pump stops due to a clogged inlet and someone has to enter the sump and clean it out! But the action of the foot valve is still required in the end because it serves to forestall any manual priming of a pump with a negative suction lift.

[jprocess]

Dear Mr.Montemayor,
Like the always i enjoyed from your deep knowledge in the field of process engineering.
Thanks a lot for your reply.
But two other questions:
1.Using foot valve is convenient for all pumps with negative suction lift?
2.When we want to calculate NPSHa for these application what value should i consider for operating pressure term in NPSHa formula?
Thanks in advance.
Best Regards.

[joerd]

2.When we want to calculate NPSHa for these application what value should i consider for operating pressure term in NPSHa formula?

Rather than trying to explain it here, please visit Fluide design pump tutorial and NPSH explanations.

[Art Montemayor]

Great comment and recommendation, Joerd!

Why try to outdo Jacques Chaurette and his great/simple explanations? That website is one that all engineers - especially students - should visit, read thoroughly and download for future reference. I have been reading his writings for years now, and I enjoy his site even more lately since he started to include a lot of fotos of himself, his family/friends, and his hobbies.

The guy is down-to-earth and makes fluid flow/transport look easy and enjoyable.