Relationship between pump discharge piping length and pump flow

[NovaStark]

Good day all,

I am trying to find any information I can on how the length of piping of a pump's discharge affects its performance/operation.

Consider a single stage OH type pump which takes atmospheric hydraulic oil and pumps it to a compressor at 180 psi. Suction enters the pump horizontally and discharges vertically.

If the distance between the discharge nozzle and outlet piping valve is 2 feet, how would this affect the performance/operation of the pump if the distance is reduced to 1 ft or even 6 inches ?

The main reasoning would be to remove as much air pockets as possible in the piping which is currently affecting the pump's performance.

 

[Artisi]

Virtually unmeasurable,
as for air pockets - please explain, as pipe length has nothing to do with air pockets.

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.)

 

[moideen]

Artisi : the long suction pipe(not discharge)cause to create air pockets?

 

[Artisi]

What long suction?

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.)

 

[DanTh3Man]

the long suction pipe(not discharge)cause to create air pockets?

Yes, the pump will cavitate if you don't have enough NPSH. Depending on the pump the friction losses in the suction piping can be enough for the suction head to fall below NPSHr.

 

[Artisi]

The OP is asking about discharge pipework, no mention of suction pipework.

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.)

 

[DanTh3Man]

Yeah, they did a flip-flop. You already answered their original question.

 

[NovaStark]

@Artsi:

The current issue is that we have a pump that is fed from an atmospheric reservoir such that the pump is self primed i.e. flooded.

However when we start the pump, the discharge takes about 20+ sections to build to the correct discharge pressure(this pump is required to autostart as a standby unit).

The discharge piping is about 2 ft long from the discharge valve, so about halfway up is a 0.5 inch line that goes back to the reservoir (continuous flow while pump is online). Based on the temperature we can measure along the pump, the entire pump is around the same temperature as the suction piping (hot fluid) however, the discharge above this 0.5 inch line is ambient. Thus we are considering the fact that the 0.5 inch line is bringing in air through this line, thus causing a vapor lock type scenario.

Decreasing the length of the discharge piping would in theory reduce the air pocket. Now this likely isn't the root cause of the issue but it may be a temporary fix until we can do more troubleshooting.

 

[moideen]

Artisi (Mechanical)5 Feb 20 08:27
What long suction?

my suction pipe is 115 meter,condenser pump. i will start new thread with this issues.

 

[Artisi]

moideen, what has 115 metres got to do with this thread.
Yes start your own thread.

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.)

 

[Artisi]

NovaStark :

Where does the 0.5" by-pass line terminate - to atmosphere in the reservoir or below the supply surface?

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.)

 

[NovaStark]

Artsi:

Unfortunately, we have no documentation for the internals of this reservoir.

Theoretically, I would think it is below the supply surface. But based on the symptoms, it appears that it may be in the atmosphere in the reservoir.

Another anomaly is the operating level in the reservoir is about 52" but the discharge piping up to the valve is about 45". So the up to there should always be full of liquid.

We know there is an internal baffle with a cut out at the top to equalize the pressure (one side with the level gauge and the other with the pump suction), so as to the level on both sides of the baffle I can't say. For the temperatures along the discharge piping, I'd have to say that the level on the side with the suction is lower (I really can't think of why apart from the inlet flow to the reservoir being less than the flow rate of the pumps).

 

[Artisi]

1.When you shut down does the discharge drain back to the reservoir?

2.Thinking the 0.5" is the key,

3. How about a non return valve on the 0.5"line.

4.if No1 is yes, how about a NRV on the discharge line at the pump discharge.

A sketch of the set up might make it a little clearer to understand.


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.)

 

[LittleInch]

novastark,

I'm trying to follow this but without a diagram / drawing /picture and a step by step description of what is happening at the start of your pump it is very difficult to follow.

I think what you are talking about is that your pump is isolated by the discharge valve? then the pump starts but because you have air in the system it takes 20 seconds ( not sections) to achieve the required pressure before you open the valve?? But where this mysterious 1/2" line is and where is goes / what is its purpose is unclear.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[NovaStark]

@ Artsi:

1. It appears that it does - again based on temperature of the piping. Based on the height of the section in relation to the reservoir, it shouldn't (?).

2. This is also what we are thinking but can't say why exactly.

3. A NRV on the 0.5" line would work but that could be a potential point of failure in the future but may not solve the root cause of the issue. If we can understand the exact mechanism and why, then we would explore those options.

4. See description for clarity.

@LI (+Artisi)

I have attached a simplified sketch of the setup (forgive my terrible drawing skills!).

So there are two pumps - A (main) & B (standby) which send oil to some machinery. Each unit has a 0.5" line that goes back to the reservoir (each has a an orifice plate with a 4 mm hole in it) - a re-circulation line. Now we don't know if the recirc lines extends into the oil in the reservoir or above the surface of oil in the reservoir. (We are thinking above because of the issue).

On the discharge of each pump is a check valve and gate valve. So both gate valves are open and the check valve of Pump B prevents reversed flow. This discharge section is about 2 ft. long and the recirc lines are about 6 inches below the check valves.

On a normal basis, A runs. If A is to trip, B is set to autostart if the speed of A drops or if the common header pressure drops.

On recent testing of the system (we don't normally test for this, we usually just test to make sure that B autostarts), we saw that pump B took more than 20 s to reach 180 psi. So its pressure would go from 0 to 100 psi, then slowly build to 180 psi. In a real life situation, if A is to trip, B would take more than 4s to start and by that time the machinery trips.

 

[Artisi]

Really stuck for anything constructive other than:
Assume the check valves actually check - no leakage back to the reservoir?
Can you temporarily isolate the by-pass line and see if any difference in the pump performance?
How about having the bypass above the check valve?

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.)

 

[NovaStark]

@Artsi:

If the check valves were passing, then the discharge would be filled with oil and the problem would not be present in theory. But not reverse rotation in the pumps, so we think they are holding well.

We suggested isolating the the recirc line but the upper mgmt was against this idea as they assume the pump would be damaged upon start-up.

We were either thinking to actually move the line above the valve OR from my OP, move the check valve closer to the discharge.

Our suggestions seemed to be more temporary measures than determining the root cause. So if we knew for sure that something was in fact wrong with the recirc line in the reservoir, then it would be easier to sell to do the above (as there would be nothing we could do without draining down the reservoir).

Also for information, we have 2 other equivalent set-ups with the same issue - so we think it is a design flaw as we have had instances where pump A trips, pump B starts as per design and then the machines still trip. (we have no individual pressure transmitters that are trended, just one on the common header).

 

[LittleInch]

Have you got a pump curve you can post? Something odd going on here.

Nice drawing by the way.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

With 2 systems with the same problem, it's reasonable to assume a design or application fault.
Cannot understand the problem of a temporary isolation of the by-pass, the pump is only starting against a shut check valve not against a closed discharge valve.

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.)

 

[NovaStark]

@ LI: I have attached the pump curve with any identification blocked out.

@Artsi: I think the issue being raised is that the 0.5" line might be to prevent overheating of the pump so during start-up it might "fail" as it is put. So if we can conclusively prove the line is the issue or isolating won't cause any problems.