Hot Water Circulation Pump.

See attached data sheet.

Your flow and developed head look right for this pump.
But the inlet pressure looks very low for hot water.
The design shows the inlet at 6bar.
You are likely either flashing to steam in the pump inlet (cavitation) or getting steam in the inlet piping and then trying to pump the two phase fluid.
Which this pump will not like.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

What is the minimum normal height of water (with reference datum as pump suction nozzle centre line) in the expansion drum just upstream of this pump ?
Can we see a P&I D of the suction line to this pump and what is the total length of this suction line ?

See thread338-515363 for previous basis discussion on the topic.

Hi,
Check for cavitation (NPSHa) and oxygen content in the water.
Attached a document to support your work.
Pierre

Dear George,
Attached is the P&ID for the Hot Water Circulation Pumps. The expansion tank level is at 1,463 mm, and the nitrogen pressure is 1.3 barg. Also attached is the P&ID for the Hot Expansion Tank.
As of this morning, the operating conditions are:
• Suction pressure: 1 barg
• Discharge pressure: 4.9 barg
• Temperature: 101°C
• Flow rate: 180 m³/hr

P&I D is not loading up - can you reload ?

Please see attached.

Hi,
I cannot open the document.
Pierre

Please try this.

Suction pressure at pump if there were no frictional loss = 1.3 + (1.46*0.92/10.2)+1.013 = 2.44bar abs = 27.1m of head abs = 1.44barg
You report suction pressure as 1.0barg, so frictional loss = .44bar = 4.87m head loss; I find that a little difficult to believe, even when you have a check valve in the suction line.
BTW, I suspect this 3inch line leading to the pump is not the actual pump suction line. This expansion drum must be sitting on a tee in to this suction line. Pls confirm ? You couldnt be running 180m3/hr through a 3inch line.
In any case, if we assume actual suction line loss = 1.0m, then remaining head = 27.1-1 = 26.1m abs. Subtracting vapor pressure of hot water at 100degC,which is 1.013barg = 2.03bar abs = 22.5m, then net head remaining at pump suction = 26.1-22.5= 3.6m.
The pump datasheet states a minimum NPSHr of 7.3m, so you are short by 3.7m, which is the reason everyone is telling you this pump is cavitating. To stop this cavitation, tank N2 pressure should be increased to say 2.0barg.

Pls also note that an expansion drum sitting on a tee in to the pump suction is not a good idea, since most of the vapor bubbles in the return hot water, if any, will remain un degassed in this stream. Vapor in the return hot water can result if you have a tube break in the crude - HW heat exchanger, particularly if the crude side pressure is greater than HW pressure at this heat exchanger. Why have you reduced HW expansion drum pressure now from 6barg to roughly 1.3barg?

Dear George,

Thanks for the detailed explanation.

I will gather all additional information and provide responses to your questions/clarifications shortly.

Very much appreciated.

The OP neglected to mention this is a duplicate post from here where we've been saying the same thing,....


S.G Double posting is not normally allowed so please ask the mgt to move it and at the very least, add a link back to your original post.

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

Apologies for that, LitteInch. I will keep this thread/post and add a link to the original post.

The 3" line just looks like the make up water not the main pumped line.

The other thing is that the flow reported and the pressure differential don't match the pump curve. At 180m3/hr, you should have over 5.5 bar differential. You have less than 4.

So either the pumps are busy cavitating merrily or trying to pump two phase fluid.

I've already told you your system pressure is far too low. Find out why someone has set it so low, then raise it to 3 bar or more if you want or need to operate at 140C as per the pump days sheet

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

Dear LittleInch and George,

You are correct. The 3-inch line is for makeup water rather than the main pumped line. The main suction header size is indeed 14 inches. After reviewing the trends, I agree with both of you that this is a case of cavitation. From my analysis of the trends, I noted that prior to the long shutdown, the suction pressure was 1.7 barg and the discharge pressure was over 7 barg.

I will provide the remaining clarifications shortly.

Stand next to the pump and listen

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.

I have requested that the related thread on the corrosion engineering forum be closed, and that all discussion is directed to this more appropriate location.

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.

Dear George,

The vapor pressure of water at 100 degrees Celsius is approximately 1.013 bar absolute. Can you please double check your calculations above?