High pressure water closed loop process with gas generation and priming 1

[Eric_ele]

Dear all,

We face a recurring problem in some of small electrolysis installations during depressurization.
This systems operate a close loop including
a pressurized water/gas separating tank up to 40 barg
a centrifugal pump usually located at a lower elevation than the tank
some filters
a gas generation cell, etc.
a two phase flow return line back to the gas separator/ tank
a pressurised gas piping system with pressure control.

This water loop is always filled with water, and operates fine on many systems.
During shut down,
gas generation is stopped,
the remaining gas is vented through a control valve
the pump operates until at least pressure is returned to atmospheric, to prevent emptying of the filters and cells (and overfilling of the tank) due to bubble expansion.

But with some pump manufacturers, the pump will fail during shut down, even with the same type of equipment. (usually centrifugal).
All ideas to improve the requirements / selection of pump are welcome.

Thank you very much.
Eric

(updated)

 

[btrueblood]

Fail how?

 

[Eric_ele]

Btrueblood,
Thank you for you reply.
The last 2 failed pumps were multistage centrifugal self priming pumps.
Failure usually happen during the first trial runs, and results in damaged shaft seals and twisted shaft.

We previously had several screw pumps fail (with similar damages) on another installation.

Eric

 

[btrueblood]

Did the pumps fail due to ingestion of debris and binding the impellers?

Do the pumps only get turned on in the shutdown phase? If so, are they hard-started or brought up to operating speed and pressure slowly to avoid shocks?

Any signs of overheating on the pumps, i.e. were they essentially operating dead-headed for some period of time, or is there a minimum flow bypass for them?

And above all else, have you contacted the pump manufacturer to see what they have to say?

 

[Eric_ele]

Btrueblood,
Thank you again for you reply.
Here are the answers:
Debris: no the system are always cleaned up to our best before being started. It pumps only water, and we never found such debris in the pumps.
Operation : the pumps are started when the system is at rest (atmospheric pressure), inline with the downstream equipment, and with all valves open.
Then they should operate for hours, days, months, until the system is returned back to atmospheric pressure in the tank depending on demand.
Overheating: no signs of overheat.

Failed pump manufacturers support :
for screw pumps, they sent new ones with remachined screw. Same result. We concluded to go back to centrifugal.
for centrifugal, they say we have to install a non return valve at discharge.

Some pumps have operated this way for >10 years with daily stop/starts.

 

[georgeverghese]

Is the pump stopped during shutdown - depressure mode? If it is, then reverse flow is possible since (a) there is a gas cap at the generation cells to sustain the reverse flow and (b) there is no check valve on pump discharge.

 

[Eric_ele]

Dear georgeverghese,

Thank you for your answer.
We stop the gas generation first, then wait a few so that bubbles can escape the water loop, and then we start depressurisation of the tank by venting.
The pump is powered at least until the tank pressure has returned to atmospheric.

Eric

 

[btrueblood]

Does the pump ever dead-head against a closed valve, or alternatively, does it drop below the rated minimum flow rate? If there is a recirculation valve to maintain minimum flow, is there a temperature reading for the inlet, and does it ever rise beyond the allowable limits? Does the pump cavitate or choke/stall when the pressure drops - this might be hard to determine other than to know the flow rate and output pressure vs. pump rpm, but if the last stage of the pump starts to stall/surge you could see high transient loads on the shaft, similarly if the pump goes in/out of cavitation that puts 2-phase flow at the inlet stage.

 

[georgeverghese]

Is this a variable speed pump, with pump speed dropped down to min speed during the depressure? If so, then reverse flow may still occur if at the min speed, the pump is incapable of producing the head corresponding to the pressure at the gas generation cells.

 

[LittleInch]

A flow diagram with pressures and control valves would help here as it doesn't make a lot of sense to me at the moment.

If this is a closed loop system, why do you need a multi-stage self priming pump? Normally the issue with loop pumps is that they are high pressure but low differential head as the frictional losses in the loop are low.

What power pumps / differential head and flow are we looking at here?

What pressure does the separator ruin at compared to the gas generation cell?

do you have any pressure or flow control valves anywhere in the liquid system?

If you don't have NRVs on the system how do you ensure no backwards flow?

This seems to indicate some surge pressures or high impulse loading onto the pumps somehow to get this sort of damage.

when this gas generation stops how fast does it stop? I'm wondering if the bubbles are then collapsing and you're getting intense pressure spikes as the bubbles collapse?



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

 

[Eric_ele]

btrueblood, georgeverghese , LittleInch,

Thank you very much for your answers. I will try to answer everything.

Does the pump ever dead-head against a closed valve : Not up to now.
Does it drop below the rated minimum flow rate : some systems have no flow switch or transmitter after the REX that it proved not useful in the past.
If there is a recirculation valve to maintain minimum flow : manually set by the operator
Is there a temperature reading for the inlet : yes
Does it ever rise beyond the allowable limits : not normally. High high alarm fires shutdown procedure.
Does the pump cavitate or choke/stall when the pressure drops : hard to tell without FT. Depressurisation speed has been adjusted on equipped processes and their pump does not stall. No traces of cavitation damages.
Is this a variable speed pump : no.
Why do you need a self priming pump? : pump supplier said it would be better in case the pump loses prime due to gas in the loop.
why do you need a multi-stage pump : filtration includes a fixed bed of absorbing media.
What power pumps / differential head and flow are we looking at here? : we run 1 kW pumps as well as 100 kW pumps. Head is around 1 to 5 bar, flows from 1 to 200 m3/h depending on installation.
What pressure does the separator run at compared to the gas generation cell? separator is directly fed by the gas generation cells, in which pressure drops by 1 bar or less.
If you don't have NRVs on the system how do you ensure no backwards flow? The outlet of gas generating cells is designed and checked to be wide open, having a 2 mbar pressure drop at full flow including gas) so that we do not except (or have witnessed) backward flow.

This seems to indicate some surge pressures or high impulse loading onto the pumps somehow to get this sort of damage : sometimes pump only suffer from seal damage.
When this gas generation stops how fast does it stop? Fastest scenario : drops to 0 in milliseconds (safety stop). But already existing gas bubbles do not collapse. Only the gas that is still attached to the electrodes can be converted back to water at a rate decades smaller than generation.

Eric

 

[keyepitts]

Maybe you are looking in the wrong direction; what about piping loads? Could heat-soaking or something cause unusually high piping loads to be transferred into the pumps? Just thinking out loud here.