Hot Water Circulation Pump.

[S.G]

Dear Team,
Please find the attached datasheet for a hot water circulation pump.
We have experienced failures and loss of metal in the impeller, as well as in the first 5 inches of the discharge piping.
The operating conditions are as follows:
• Suction pressure: 1.2 barg
• Discharge pressure: 4.9 barg
• Temperature of fluid: 100°C
• Flow rate: 180 m³/hr to 200 m³/hr
Could you please confirm whether we are operating the pump outside its recommended operating envelope?

 

[georgeverghese]

Suspect it is not a good idea to completely shut in the standby pump when not in use. Allow for some trickle flow of fresh inhibited HW continuously in the reverse direction. So leave the inlet valve open, install a DN20 bypass ( with needle or globe valve) across the discharge valve and drill a small hole in the discharge dual plate check valve leaf. Inspect the check valve for proper action every year or so.

 

[LittleInch]

SG,

Despite many posts I don't think we're that much closer, but looking at the data again, Your pump inlet pressures are just too low.

Doing this calculation properly min 0.8m suction is 19.4m head based on hot water density
The vapour pressure at 105C is 12.8m

So NPSHA is 6.8m vs NPSHR of 7, even at the supposed 188m3/hr. You need probably at least 3m margin to avoid cavitation to this pump but maybe more due tot he additives so low pressure / cavitation is still your primary causation of damage IMHO.

Also I still don't understand why your differential pressure has fallen so much. If the extra flow isn't going round the spare pump then how is this happening? Are all the pumps suddenly going backwards? Is there a short circuit somewhere not shown on the P&IDs?

The issue now is raise the inlet pressure / static pressure in the tank, but you won't know if its made a difference or not..... Until at least three months.

Have you checked / stripped down the currently working pumps yet? Maybe the low DP is because the impellors have all been eaten away??



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

 

[S.G]

Hi LittleInch and George,

Here is the feedback from the chemical supplier:

“The chemical (NORUST SC 47Z UAE) is unsuitable for closed systems or applications at temperatures higher than 80°C. At higher temperatures, ammonium bisulfites decompose, emitting various gases and forming other compounds.

While the specific decomposition by-products depend on the reaction conditions, they typically include ammonia (NH3), sulfur dioxide (SO2), water (H2O), and possibly some other substances.

The decomposition reaction above 80°C can be represented as: 2NH4HSO3 (aq) -> NH3 (g) + SO2 (g) + H2O (g) + NH4HSO4 (aq). This indicates the release of gases, including ammonia (NH3) and sulfur dioxide (SO2).

These additional gases contribute to the overall gas content in the water, thereby impacting the NPSHa.Thoughts?

I will respond to your post above tomorrow.

 

[EdStainless]

Yes, additional gasses will make things worse for you.
And it makes me wonder what the pH is at your service temp and pressure?

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P.E. Metallurgy, consulting work welcomed

 

[S.G]

Hi Ed, we have been monitoring the water pH value. The PH value is ranging from 7.5 to 8.4.

 

[georgeverghese]

You wont be doing any better with hydrazine with the current setup, since it decomposes into N2 upon contact with dissolved O2 in HW. NH3 and SO2 are very soluble in water; NH3 and SO2 vapor will form only when quite a bit is already in solution, even at 80degC, 1.2 barg. Ways of controlling accumulation of dissolved So2 and NH3 in this HW would be
a)to trickle purge out some HW from the circuit continuously or intermittently, so you get fresh makeup from the expansion drum
b)Obviously the amount of vapor in the closed loop-low pressure section of these very soluble components would decrease with increasing pressure, so here is one reason to increase operating pressure in the expansion drum.
c)Relocate the site where you inject O2 scavenger (which I presume is somewhere in the closed loop )to the makeup drum, and inert the makeup drum with N2 - this will work okay only with hydrazine as scavenger, and may or may not with amm bisulfite( considering pressure and temp in makeup drum vs pressure and temp on HW closed loop pump suction). I would go with N2H4 if you ask me.

N2H4 + O2 = N2 + 2H2O

This is among the reasons why it is preferable to have an inline expansion drum, with HW return going into the drum. There is no way you can modify this drum now to enable a 14inch exit line.

 

[EdStainless]

But is that measured pH at room temp and ambient pressure?
If so then it isn't what your system is seeing.

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

 

[S.G]

Dear All,

We concluded the failure analysis of metal loss in the hot water generation system.

Root Cause - Chemical Compatibility and Behavior at High Temperatures:

It is evident that the chemical (NORUST SC 47Z UAE) is unsuitable for closed systems or applications at temperatures exceeding 80°C. At higher temperatures, ammonium bisulfites decompose, emitting various gases and solid residues.

While the specific decomposition byproducts depend on reaction conditions, they typically include ammonia (NH3); sulfur dioxide (SO2); water (H2O), and some other possible substances.

The decomposition reaction above 80°C can be represented as: 2NH4HSO3 (aq.) -> NH3 (g) + SO2 (g) + H2O (g) + NH4HSO4 (aq.). This indicates the release of gases, including ammonia (NH3) and sulfur dioxide (SO2). These additional gases contribute to the water's overall gas content, thereby impacting the NPSHa calculations.

The impact of this decomposition can vary depending on the context. A few possible impacts to consider are oder, toxicity, and corrosion.

Root Cause- Operating Conditions Review:

The combination of high temperatures, chemical decomposition, and operational conditions below the pump’s ideal curve creates a scenario where cavitation is likely. The high temperature contributes to the chemical decomposition of the oxygen scavenger, leading to gas formation which exacerbates the formation of vapor bubbles. When these bubbles collapse, they cause the physical damage observed.

- Sensitivity calculations show pumps could be experiencing cavitation during high water temp return at suction (>105°C) – high DP of 0.6bar observed over inlet strainer of Pump A and 0.4bar Pump B could be contributing to cavitation

- Inconsistencies of flow meters and Pump DP – recommend recheck of flowmeter calibration / dp checks and flushing – or there maybe clearance issue between impeller and casing impacting performance – ongoing,

- Pumps possibly operating beyond their rated case of 200m3/hr. (down the curve) observed from low pump differential - estimated flow 260m3/hr. and (9m NPSHR).

 

[LittleInch]

S.G.

Thanks for the update. Will be good to know what the end result is after you've done the extra tests on the flow meters and the pump casing.

And clean your filters....



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

 

[georgeverghese]

0.6bar = 6.8m head loss; 0.4bar = 4.5m head loss at the pump strainers alone. Strainers got bunged up with corrosion debris after recent restart ?
Would inadequate preservation measures in this HW loop at these pumps during the 4-6month long recent shutdown be part of the cause for this severe metal loss at these pumps?