Hot water circulation pumps - Metal Loss. 2

Cavitation?

Air ingress?

Crude ingress?

Or erosion caused by excess velocity?

Why the big difference in suction and discharge pressures between data sheet and operation? - suction 1.5 bar versus 6 and 5 versus 11.5??

What is the flow rate? Are you off the end of the curve? Delta P on the data sheet is 5.45 barg, but operation is 3.5 barg. BIG difference right there.

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Dear LitteInch,
Please see attached the pump curves.

Dear LitteInch,
The working flow rate is 180 m3/hr.

Dear LittleInch,
Please see attached photos of the pump impeller and discharge pipe( the first 5 inch of the discharge pipe is eroded)

Well if your DPs are correct then you're well off the end of the curve and doing about 320 to 330 m3/hr.

Plus your NPSH / cavitation curve is heading very high considering you're pumping superheated water.

Lots wrong here so not surprising your pump has given up.

Why are you not keeping the pressure higher in the water circuit?

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Also: If you get a response it's polite to respond to it.

Dear LitteInch,
I will discuss this with the operations team and get back to you.

Pretty classic looking cavitation damage on the inlet vanes for sure.

How are they measuring flow?

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Also: If you get a response it's polite to respond to it.

If you want an idea of the NPSHr needed to prevent issues just compare the vapor pressure of water at 68F to 140F.
You need to work on this system.
LI hit it on the head.

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

Static pressure is far too low at those temperatures.

You are getting flashing steam.

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Also: If you get a response it's polite to respond to it.

Dear LitteInch,

We have tested all pumps today, please see below for pump A:

Suction pressure: 1 barg.
Temperature: 80 deg C
Discharge pressure: 5 barg
Flowrate: 188 m3/hr

and yes there is a flowmeter (FIT)located at the outlet of the hot water generator. According to our operations team there has been no change to the pumps operating conditions (we have been running these for years), we see such damage after we started injecting Oxygen Scavenger and Neutralizer chemicals.

We started chemical injection on 14th of August, Pump C was replaced in October 2023 and failed again on 24th December 2023. See below:

"HWG circulation pump –C was replaced under corrective maintenance job reference number CM095936 in October 2023. Production reported to maintenance with defect raising pump leaking on 24 December 2023. While the maintenance team attended and inspected the pump on the skid and found water was leaking from the pump casing through a pin hole. Based on the criticality of the operation, decided to replace the pump with the available new stock. An arrangement has been made to make use of the pump for night for emergencies only. on 25th December followed with the laser alignment and commissioned the pump for production use. The defective pump was dismantled in the workshop and thorough inspection has been carried out, during the inspection it is found to be heavy metal loss on the pump casing, impeller. due to heavy metal loss the casing encountered a pin hole, and the water was pressuring through that. Based on our inspection it is suspected the metal loss within 3 months is an unacceptable wear rate. To our Suprise these pumps were running from past 4 years with very minimum failure rate, that to be only seal and bearing failure. The reference disassembled components photos for more clear reference explanation.

Regards,
Sohaib

Well it might be something to do with the chemicals, but why were they introduced?

Your pressures seem low to me.

The flow rate and diff head doesn't add up.

What is the fluid? The data sheet says 920 kg/m3??

If the water was aerated before, bubbles can mask cavitation.

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

Dear LittleInch,

The chemicals were introduced for corrosion control, and the data sheets relating to these chemicals are attached to this mail.

The fluid concerned is water. It is worth mentioning that the system was shut down for over six months from March 2023 to August 2023. Following the restart in August, we began the chemical injection and the first failure occurred in October.

We checked the operating conditions (pressures, temperatures, flow, etc.) before and after the shutdown, and the only thing that had changed was the introduction of the aforementioned chemicals.

Possible causes: Chemical incompatibility issues, such as cavitation because of the added chemicals. System preservation.

Any assistance/advice you can offer us would be greatly appreciated.

Attached.

What are the materials of construction for the pump?

Steve Jones
Corrosion Management Consultant


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

The damage morphology does not appear to be directly relatable to the chemical treatment. Perhaps close this thread and repost in the Pump Engineering forum to get feedback there. If you do that, don’t forget to explain to both forums so that answers do not get overlooked.

Steve Jones
Corrosion Management Consultant


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

As Littleinch and Edstainless says - its cavitation.

Boiling pressure of water at 140ºC is 2.6 barg so you basically have boiling water in your pipes at the suction pipe, but the discharge pressure is above boiling pressure - cavitation.

The pump spec says 6 barg and this would be a good static pressure. Are you sure that the lowering of the system pressure isnt a resent thing and that you are looking at the wrong cause (chemicals) because thats something you just latched on to?



--- Best regards, Morten Andersen

That is certainly what cavitation does.

Only other weirdness possible - has the water been checked to see that what was added is exactly what was intended?

We had a case where a newly manufactured vehicle was sent to get a final wash before delivery and engineering gets a call the next day that all the hydraulic fittings needed to be changed to stainless steel, about 200+ of them. We inquired as to why that was needed and the factory said every fitting was blossoming with rust.

Turns out the guy in the wash bay, instead of using degreaser and soap, used the 55 gallon drum of phosphoric acid. That sure worked. Removed all the plating from all the fittings. Great job. The drums were clearly labeled, he just thought all of them were the same and didn't bother to look at the writing.

Or how much was added?

Oxygen scav and neutraliser are normally injected as ppm, I.e. very small amounts. How much was added? To what volume?

Next is "According to our operations team there has been no change to the pumps operating conditions" Yeh, right. Ask any operator what's changed and the answer is "nothing!". Unless you've got hard documentary proof then I might be tempted to not fully believe them.

Either way, what EXACTLY are your operating conditions in terms of pressure and temperature?

And flow doesn't match the pump curve so something isn't right there either.

I just don't understand why you're operating at such a low pressure. The pump say 6 bar inlet and you say 1 bar or 1.2 to 1.5 bar
Temperature - the data sheet says 110 to 140C, hence the lower density that is the density at 140C. But you now say 80C??

Now even if truly "nothing has changed", then it might be that the chemicals have lowered the vapour pressure or removed air bubbles.

Your root cause though is that very low operating pressure. Can't you simply increase the pressure?

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

OP has now moved the thread to the Pump Engineering forum

Steve Jones
Corrosion Management Consultant


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