Recirculation line?
Recirculation line?
(OP)
New maintenance engineer here, working at a chemical plant.
Currently we have three vertical pumps for a sump in my unit. A primary always runs to maintain sump level at 40% and secondary pump kicks on when sump rises above 60%. The third is a backup. So the main issue is that the primary pump runs below or at minimum recommended flow(334 gpm) for 95% of the time, it isn't until we have rain or other dumps that allow the pumps to open up closer to their BEP (1300 gpm). All three pumps feed into one control valve.
Now there is a small 2" recirculation line that only provides about 120 gpm according to tests and actually dumps into the pumping chamber right at the suction of a pump which has its own air entrapment issues. With this poor design I am looking into sizing a new recirculation line to dump back in the settling chamber and then getting a Vfd for the primary (due to cost).
With the CV 100% open, the discharge pressure off the primary pump was 22psig. There is no other points to get a pressure reading downstream of the valve when it is loaded during normal ops.
My question is , what should my plan of attack be to size this line? Any advice or tips for a young engineer would be great. Thanks in advance!
Currently we have three vertical pumps for a sump in my unit. A primary always runs to maintain sump level at 40% and secondary pump kicks on when sump rises above 60%. The third is a backup. So the main issue is that the primary pump runs below or at minimum recommended flow(334 gpm) for 95% of the time, it isn't until we have rain or other dumps that allow the pumps to open up closer to their BEP (1300 gpm). All three pumps feed into one control valve.
Now there is a small 2" recirculation line that only provides about 120 gpm according to tests and actually dumps into the pumping chamber right at the suction of a pump which has its own air entrapment issues. With this poor design I am looking into sizing a new recirculation line to dump back in the settling chamber and then getting a Vfd for the primary (due to cost).
With the CV 100% open, the discharge pressure off the primary pump was 22psig. There is no other points to get a pressure reading downstream of the valve when it is loaded during normal ops.
My question is , what should my plan of attack be to size this line? Any advice or tips for a young engineer would be great. Thanks in advance!





RE: Recirculation line?
What is the maximum inflow to the sump during rain periods?
Are the pumps oversized for the application?
Consider changing primary pump for a lower flow unit to operate near the normal condition (95% of the time)
Size the by-pass line to larger dia. possibly with a manual control valve to allow setting during normal operating periods.(move the discharge from the by-pass line to elsewhere or discharge it well below the sump surface level)
You need to analysis the costs involved of fitting a VFD and the increased running costs (VFD inefficiency) compared to running the pump operating near to its BEP with a by-pass line in operation, you might be surprised with the outcome between the 2 options.
A lower flow unit seems to make some sense, provided the other 2 units can maintain flow during peak demand.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Recirculation line?
- Dont have the maximum inflow yet, I will be getting this tomorrow.
- The pumps aren't oversized when they need to run at full capacity, but the other 95% of the time, yes they are.
- A lower flow unit for the primary is an option, however this would also require you to purchase a spare for that smaller unit.
- Could you clarify "size the bypass line to larger dia"? I was thinking of using a 4" line and yes dumping into the settling chamber.
- VFD will cost roughly 20k (high estimate) to implement on one primary. I plan on running a plant test with a temporary 4" line to see our flow.
- I guess my other question is when sizing this recirculation line, what do I need to do in terms of losses? To ensure the fluid would still get where it needs to go.
RE: Recirculation line?
as for a stand by, why bother - maintain the existing primary in case it's ever needed.
Look at a quality electric submersible as the low flow unit.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Recirculation line?
Furthermore if a VFD can get the job done and it's a cheap option than a submersible, that may be the way to go no?
RE: Recirculation line?
And why would VFD be cheaper than a submersible, bear in mind VFD is not a panacea to cure all ill's or a poor pump selection.
You need to undertake a proper cost analysis, including the running costs of a VFD running an overcapacity pump compared to a correctly selected pump for the 95% operation.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Recirculation line?
If static lift is a big part of your pump output, vfds are often not very variable as head would drop off too fast.
Everything else artisi says I agree with.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Recirculation line?
For this application I like the automatic recirculation valve. We use these on a number of pumps and have generally had good experience with them. This really gives you the best of both worlds. You get the high capacity during rain events and protect the pumps the rest of the time.
Johnny Pellin
RE: Recirculation line?
I'm just saying if the smaller unit it went down for whatever reason, you'd be back down to running a primary in poor condition for whatever duration. I guess this would still happen if there was a VFD on one of the them anyways. Would it then be wise to still go ahead and size a larger recirc line for the other pumps if the submersible is down?
RE: Recirculation line?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Recirculation line?
RE: Recirculation line?
My hunch is that you have lots of excess flow, but very little excess head, so a VFD will probably not work.
I like the idea of a smaller pump for standard duty, and a larger recerc line for in between times.
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RE: Recirculation line?
Johnny Pellin
RE: Recirculation line?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Recirculation line?
RE: Recirculation line?
Like I said a few posts ago, search for ARV - automatic re-circulation valves. These automatically monitor the flow and without instrumentation and power, only divert flow via the re-circ line when there isn't enough forward flow.
An example is here http://www.schuf.com/products/pump-recirculation-v...
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Recirculation line?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)