resizing pumps - measured data plotted on pump curve

[vaavas1]

Hello,
I work at a power plant and have a project to replace a couple pumps used for routine washes (boiler wash pumps 51&81). These pumps have a reoccurring problem and have to be put out of service, pulled out and repaired because of vibration caused from the shaft bending - breaking the mechanical seal.
BWP #51 was replaced in 2008 from 50hp 235gpm 420ft head to 125hp 525gpm 600ft head. BWP #81 was replaced in 2010 (a one for one replacement: 125hp 525gpm 600ft head). Im not 100% sure why they were replaced, but I think both were replaced due to corrosion. The water used for these washes is from a stream and corrodes the inside of the pipe (and probably the pump column). Since the BWP# 51 replacement in 2008, maintenance has pulled out and repaired this pump in 2012, 14 and 15. Since the BWP# 81 replacement in 2010, maintenance has pulled out and repaired this pump In 2012, 13 and 14.
I attached the pump curve for both pumps with the measured head and flow plotted on it to see where it is operating relative to its pump curve. My question, is this even possible to get data like this? I was expecting the points to be riding along the pump curve, but its not, it looks like the head is held constant no matter what flow it provides. Im using an external flow meter attached to the supply and recirculation line to the pump (measure just one pump- only one pump is running at a time). The head is being measured from the discharge of the pump, by a pressure data logger.
The system setup, is a 6" supply line and a 2" recirculation line with a control valve set at 200 psig. These pumps are vertical centrifugal pumps, getting the water from a pit about 8' deep (constant supply of water). Different washes require different flow demands, so it's not just one flow-head needed.
I also attached the pump outline, and a plan view sketch showing where the data loggers are set up. The piping was corroded and the external flow meter wouldn't read any flow, so I replaced portions of the pipe to be able to measure flow. Also, theres a question of if the flow meter is reading accurate flow, the 2" line the flow meter would have problems getting data and I spent many hours fiddling with the sensors mounting-remounting in order to get a reading. the sonic velocity that the flow meter gave to me is:
6" line: between 1482 (m/s) - 1520 (m/s) [average 1488]
2" line: between 1295 - 1778 (m/s) [average 1407]
any help / advice would be greatly appreciated. thanks everyone!

 

[BigInch]

Yes head is held constant by the boiler pressure that the pump is supplying. Boiler pressure stays relatively constant no matter what flow the pump is making.

 

[vaavas1]

Thanks for the reply.
These pumps supply wash water for cleaning the fire side surfaces of the boiler tubes, condenser and air heaters. When washing the boiler tube surfaces, the boiler is not running. The pumps are not part of the boiler, the pump and piping is an external system used to clean the boiler/air heater / condenser.

 

[BigInch]

In that case I note that 400 ft of water is roughly equivalent to 200 psi. It may be that you're on recycle when the discharge pressure is less than 200 psig and you can make any flow rate when discharge pressure is above.

 

[georgeverghese]

Min permissible continous stable flow is shown to be at some 770ft of head for these new pumps, which translates to 330psig, so why have you got the recycle PCV set at 220psig ? 220psig seems to be appropriate as the recycle PCV setpoint for the old pump 51.

If all you require is some 200psig, then why were the pumps upgraded in terms of delivery pressure? Seems like power is being wasted with this needless recycling / pressure letdown?

Not familiar with these liquid flowmeters that use sonic velocity to infer flow - are these ultrasonic flow meters?

 

[someguy79]

It seems like you're asking if the data you provided in the plot is reasonable.
From here it's hard to say whether or not your data is accurate.

As others mentioned, it appears your system is operating at a pretty consistent pressure.

If your pumps are actually operating that far from BEP that often, you should be seeing some indication of that as vibration, cavitation, recirculation, and/or increased wear.

Where is the pressure being measured?
How are the control valves set to operate?
Do they modulate based on upstream or downstream pressure?

 

[chicopee]

When was the last time external flow meter was recalibrated?

 

[bimr]

No, it is not possible. The pump performance should follow the pump curve. If the performance does not, then there is some type of problem with the installation.

Don't assume that the pump was installed correctly in the first place.

You may have a problem with the pump suction.

 

[vaavas1]

[highlight #FFFFFF]Thank you guys for the time you spent reading over this. Listed below are my responses, I did my best to try and answer everything. If I didn’t answer something, let me know and I’ll respond with more info to hopefully clear the air.[/highlight]

BigInch (Petroleum) 28 Sep 15 22:59
In that case I note that 400 ft of water is roughly equivalent to 200 psi. It may be that you're on recycle when the discharge pressure is less than 200 psig and you can make any flow rate when discharge pressure is above.
[highlight #729FCF]The recirculation line and control valve, the control valve is open and closes to increase/maintain the discharge pressure. The water is coming out of the recirculation line pretty turbulent and aerated.[/highlight]

RE: resizing pumps - measured data plotted on pump curve
georgeverghese (Chemical) 29 Sep 15 01:00
Min permissible continous stable flow is shown to be at some 770ft of head for these new pumps, which translates to 330psig, so why have you got the recycle PCV set at 220psig ? 220psig seems to be appropriate as the recycle PCV setpoint for the old pump 51.

If all you require is some 200psig, then why were the pumps upgraded in terms of delivery pressure? Seems like power is being wasted with this needless recycling / pressure letdown?

[highlight #729FCF]I could be wrong, 220 psig setpoint is an attempt to keep the pump from going over the maximum flow. Majority of the time, the discharge pressure is low, in the 180-200 psig, which is going toward the max flow. I have never seen the discharge pressure in the 250-330 psig range, I think its because the recirc line provides enough flow for the discharge pressure not to get close to that and deadhead.[/highlight]
Not familiar with these liquid flowmeters that use sonic velocity to infer flow - are these ultrasonic flow meters?

[highlight #729FCF]Yes, these are ultrasonic flow meters. Here is the link of a video of the instrument and the performance:

https://www.youtube.com/watch?v=dZ1teWDg77k

[/highlight]
[highlight #729FCF]For each reading, it gives the sonic velocity value as well. The Siemens Instruction videos lists comparing the sonic velocity of the fluid and temp value with the flow meter value, if they are close to each other, then it should be an accurate reading. The only thing, is I don’t know the relationship between how far off you can go from the sonic velocity compared to how much it affects the velocity reading.[/highlight]

RE: resizing pumps - measured data plotted on pump curve
someguy79 (Mechanical) 29 Sep 15 01:01
It seems like you're asking if the data you provided in the plot is reasonable.
From here it's hard to say whether or not your data is accurate.

As others mentioned, it appears your system is operating at a pretty consistent pressure.

If your pumps are actually operating that far from BEP that often, you should be seeing some indication of that as vibration, cavitation, recirculation, and/or increased wear.
[highlight #729FCF]Vibration, indication of this is from the shaft eventually bending and the mechanical seal breaking. Once the mechanical seal between the motor and pump – where the shaft is exposed busts, water starts to spray out.
Cavitation: I think I can hear the water cavitating, but not 100% sure.
Recirculation, water is coming out of the recirculation line pretty violent (not sure if you were referring to the recirculation in the pipe or recirculation within the bowl-impeller).[/highlight]

Where is the pressure being measured?
[highlight #729FCF]The pressure is being measured at the discharge of the pump.[/highlight]
How are the control valves set to operate?
Do they modulate based on upstream or downstream pressure?
[highlight #729FCF]They are taking pressure at the location of where the recirculation line is connected to the main line. They are normally closed and open once they hit the setpoint (operations moved this setpoint from 220 to 200 psi, sometimes to 175psi). Each control valve is controlled by a pneumatic pressure controller. The pressure controller has the process input pressure, then feeds the bottom of the control valve diaphragm to open /close the valve with 35psi instrument air. Its normally closed with a spring to push the control valve shut.[/highlight]
RE: resizing pumps - measured data plotted on pump curve
chicopee (Mechanical) 29 Sep 15 02:36
When was the last time external flow meter was recalibrated?
[highlight #729FCF]The flowmeter was purchase in January 2015, it hasn’t been recalibrated since I’ve had it.[/highlight]
RE: resizing pumps - measured data plotted on pump curve
bimr (Civil/Environmental) 29 Sep 15 04:33
No, it is not possible. The pump performance should follow the pump curve. If the performance does not, then there is some type of problem with the installation.

Don't assume that the pump was installed correctly in the first place.

You may have a problem with the pump suction.

[highlight #729FCF]Thanks, im not sure how to check the pump suction since its in the pit.[/highlight]

 

[bimr]

Pump suction may be plugged. Drain pit to inspect.

 

[georgeverghese]

If you are running at less than 220psig on Pd, then there would be no recycle - okay.
Ultrasonic flow meters have very good turndown but dont do well on 2phase flow.
Suggest we check the pump installation for adequate submergence to correct the problem with air entrainment into the pump - the Hydraulic Institute standards has illustrations on submergence and pit dimensions from recollection.

There may be many reasons for the seal failure - shaft vibration or air entrainment and / or air lock in the seal coolant supply / return line on this vertical turbine pump would be my guess.

 

[LittleInch]

varvas1,

From what I can figure out from the posts, your pump is running at max flow continuously. Setting your bypass at a pressure rating for a centrifugal pump is a poor design and results int he issues you are hiving.

I can't work out from your description and drawings how you actually control the flow rate in the 6" line.

Not that it matter because your system is just working flat out contantly.

At your lower flows, I reckon you're doing about 25m/sec in your bypass line - not surprising it's rattling a bit.

Centrifugal pumps are essentially constantish pressure units. The bypass via pressure is therefore a bad idea. Look up ARV valves. I think they will suit your application much better.

There is a fundamental mistake in how to control these pumps going on here which you need to correct.

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

 

[vaavas1]

The 6 " line flow is not controlled. There is only the recirculation line with control valve. The 6" line is the supply for the airheater washes, condenser washes, and inside the boiler washes. Under normal operating conditions, When our maintenance needs to do a wash, they tell operations to turn a boiler wash pump on. this is left running continuously while they use the water intermittently.For the condenser and boiler washes, there are pipe connection points our maintenance crew connect their hoses to and do their wash. For the airheaters, there are gate valves at the end of the pipe that is opened and closed to do the wash. But now, one of the pumps is running 24/7 because its being used as backup to maintain pressure in our fireline.

So you are thinking to put an automatic recirculation valve? I don't have much experience with this so I know im probably wrong. ARV's look like they do the same thing as the recirculation line with a control valve.

 

[LittleInch]

The point about an ARV is that it on blocked flow it only re-circulates the minimum flow. Your set up currently recirculates the pump MAXIMUM flow continuously through a 2" line at very high velocities. The pressure would be higher as it would then cliumb up the pump curve.

This (your current setup)is a bad idea and shows little understanding of how pump and system need to be joined together properly.

Much better to install an ARV if you have long periods of no / low flow and then regulate the pressure d/s the pump if you need to restrict the discharge pressure to a certain level.

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

 

[vaavas1]

thanks again everyone, i'll look into this and hopefully a year from now, have something installed that'll solve the problem.

 

[someguy79]

I agree with LittleInch on this. It sounds like the controls you have in place conspire to put the pump near runout all the time.

An ARV should work fine.

If you're pinched for cash in the short term, a properly sized restriction orifice can be put in the recirc line. An orifice is cheap now, but expensive in the long term as you'll probably end up wasting more energy using it.

Good luck.