BFP PUMP SERGING AND IMPELLER CUTTING-OFF 3

[ryuchangmyong]

Let me say about problems of BFP Pump, its rated specification is follows.
1) rated flow : around 500m^3/hr
2) Min. flow : 200m^3/hr
3) Discharge Head : 240 bar
4) Centrifugal Type and 8 Stage
5) NHSPr : 8m
6) NPSHa : 16m
7) Test water temperature : 20 oC
8) Operation water temperature : 170 oC

1. Current Situation
When we get this BFP pump operation, we found so serious noise and vibration in a discharge ppiping line and it also appears on suction piping.( Currently, we call this phenomenon as Pump serging)
Currently, we guess that this caused by lower NPSHa at suction piping where deaerator was located at 16m height above BFP pump, In there are 3 sets of BFP pump, one of them is little bit better than others, so we believe that this was caused by NHPSa because each BFP head loss in suction line is different, thereby, short piping where located BFP is better relatively.
When we get pump test performence curve as attached file.

2. My opinion is follows at current pump operation
1) This pump should be getting right side inclined curve basically, so flow have to be decreased as much as head loss, otherwise, excessive head could cause suction head loss and thereby, serging occurs in suction line due to excessive pressure accumulation at min. flow.
and then this energy accumulation cause water hammering not only suction line but discharge cline together.

3. Query over this situation
Is it possible to get impeller cutting off in order to have inclined curve right side against parallel line between Min. flow(200m^3/h and rated flow(500m^3/h)
And how much we can expect decreased discharge head and flow rate at 5% Impeller cutting off.

 

[LittleInch]

ryuchang,

You were warned above that what you were doing was not a recommended course of action to solve the issues you face.

I think you have now proved the old proposition which is that it is rarely the pump which is at fault, more the manner in which it is used and controlled.

It was clear to most of us that below minimum flow and in your case approx. 200m3/hr, the pump was unstable, especially when operated in parallel with other pumps. You don't say how or where the flow is controlled and unless and until you can determine what your system curve looks like, I don't think you'll get very far.

You seem to have made things much worse and there is little I can suggest other than to have someone experienced in BFP system design (pumps, piping, controls) etc look at this and provide some planned modifications.

Sorry, but you sometimes have to admit you've made an error early on and then propose a way out rather than blame someone else.

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

 

[1gibson]

Not only that, he trimmed 7 impellers when he probably could have trimmed 1 or 2 of them (as previously mentioned) so that probably at least doubled the rework costs.

 

[LittleInch]

Seven new impellors won't be cheap.

Measure twice cut once come to mind....

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

 

[ryuchangmyong]

Dear All

I missed one thing to give you additional information, minimum flow is controlled by ARC valve, I also believe that the initial force of ARC valve would occurs chattering at Min. Flow.
Actually, previous P. Curve was modified as attached file. Surging range slightly moved to more flow rate against before trimming.

 

[Artisi]

I would add to Littleinch's comment --- VERY rarely is the pump at fault.

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.)

 

[MJCronin]

TAMU is a good source for papers discussing some fundamental BFP limitations and issues.

http://turbolab.tamu.edu/proc/turboproc/T5/T5pg45-54.pdf

I still maintain that vibrations and surging (note the spelling)are due to improperly designed suction piping. (You will note that the pump has two elbows just upstream of the suction nozzle and does NOT have an adequate straight length of piping feeding into the suction.

There are safe operating ranges (both minimum and maximum flow) for all centrifugal pumps. Again, have you contacted the pump vendor ?

Are you sure that the pump was not repurposed from another installation ???

It's time to pay for an expert and accept the cost of a suction pipe

MJCronin
Sr. Process Engineer

 

[LittleInch]

Can you attach a system sketch or drawing of this arrangement (i.e. not an iso or 2D model) as I don't understand where this ARC valve is. If one is fitted, why is the pump operating below 200m3/hr??

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

 

[TenPenny]

If there's an ARC valve, I'm assuming this flowrate is measured downstream of the ARC valve. I'd suggest you have the valve/valves serviced, and find out if they are properly sized/matched to the pump. I expect that 'chattering' is related to the valve. Some ARCs are good at modulating, some are not.

 

[ryuchangmyong]

Dear ALL

As I said previous, for results of Impeller trimming. Let me say what is going on after trimming.
Actually, this 8 stage BFP pump impeller was trimmed about 2% diameter from 4th impeller to last 8th impeller except but 1st to 3rd. the results of this trimming was little bit good aganist before trimming, so Surging and hunting in Min. flow rage was disappear. and total discharge and flow as well was decreased as much as impeller trimming. however,
1) pump head at bleed line was increase where located at 3rd to 4th impeller.
2) Electric Current is hunting against before impeller trimming.
I guess that decreased circumference velocity(Dynamic Pressure) at 4th impeller turn into static pressure, finally this get increased bleed line head.
Are there any this kind of experience or how to explain this situation in pump theory, pls let me know.
I don't have any idea why current is hunting after trimming but flow and head is not hunting.

 

[ryuchangmyong]

Dear ALL

Against my previous post, after trimming BFP Pump 4th to 8th as much as 2%, we are now facing another problems of Bleeding flowrate that decreased to 60m^3/h from 70m^3/h and discharge head to 50kg/cm^2 from 60kg/cm^2, Are there any idea for explanation why this bleeding flowrate was decreased. Actually, this BFP pump discharge head was decreased to 220 Kg/cm^2 from 240 Kg/cm^2.

 

[gempump]

You know different impeller diameter with different pump performance curve.There is no comparability with different impellers.You have to reselect the correct operating duty point from curve,pump speed,head,motor power,or your pump will not run normally.
Why not trying to change the pump speed rather than trimming impeller ?Most of our customers never trim impeller themselves.Not good for the service life.

Heavy-Duty Centrifugal Slurry Pump Manufacturer

http://www.gempump.com

 

[LittleInch]

ryichang,

Can you get your story straight?

Post from 9th November "We got this BFP pump impeller trimming from 2nd impeller through 8th impeller leaving 1st impeller"

Post from 26 Nov "after trimming BFP Pump 4th to 8th as much as 2%.."

post of 19th Nov "pump head at bleed line was increase where located at 3rd to 4th impeller"
Post of 26th Nov "and discharge head to 50kg/cm^2 from 60kg/cm^2" i.e.decrease in pressure?

Now if the first one is correct, then your bleed after third impellor will reduce. but has it reduced?

This post is getting more confusing as time goes on rather than less....

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

 

[ryuchangmyong]

Dear ALL

During posting this issue, there was some changes, the correct one is follows.
1. Post from 26 Nov "after trimming BFP Pump 4th to 8th as much as 2%.."
2. Post of 26th Nov "and discharge head was decreased to 50kg/cm^2 from 60kg/cm^2".

 

[Artisi]

Starting to sound like "The wheels on the bus go round and round all day long"
Suggest the OP reviews the complete story and edits all the data to come up with a coherent story on the actual problem and changes----alternately employ a competent engineer to undertake a thorough engineering study of the problem and make some fact based suggestions.

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.)

 

[ryuchangmyong]

Thank you all for the help.