Pumps
Pumps
(OP)
I trust you are well.
I have just started at this big boiler making company and there is a problem we are trying to fix with regards to water level in the boiler. We are looking into installing a differential pressure cell across the pump, such that the head of the pump can be changed as the boiler pressure modulates. This is to ensure that pump head is always greater than the boiler pressure so that there is flow at all times when required. I am working on a 11bar boiler as the test case. Feedwater tank is usually 3m tall. I do not know how to proceed from now on. I will appreciate any help on this
Regards
IsaacNewton4
I have just started at this big boiler making company and there is a problem we are trying to fix with regards to water level in the boiler. We are looking into installing a differential pressure cell across the pump, such that the head of the pump can be changed as the boiler pressure modulates. This is to ensure that pump head is always greater than the boiler pressure so that there is flow at all times when required. I am working on a 11bar boiler as the test case. Feedwater tank is usually 3m tall. I do not know how to proceed from now on. I will appreciate any help on this
Regards
IsaacNewton4





RE: Pumps
But what you also need to know is how much water flow you require and at what pressure, particularly if you are building this rig to service different size boilers. In order to select a pump at a minimum you must have a flow at a particular head or pressure specified.
I don't think you need to have a differential cell but merely a pressure and preferably a flow sensor such that the pressure and flow and the speed of the pump varied as required to suit the change in operating pressure.
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
RE: Pumps
Also think about how you control this flow. Normally boiler feed pumps work on maintaining a fixed level in the boiler. Do you intend to maintain a fixed level, hence a possible wide swing in flow or start at a low level then inject water to a higher level and switch off? Makes a big difference.
For a single speed centrifugal pump you're probably looking at a range in flow of around 40 - 110% of rated duty at a pressure which could vary by +/-10% from your rated duty ( i.e. "normal" duty)
If your flows or pressures are wider than this then you need to look at two pumps in parallel or possibly some sort of VFD, but these are normally not very good for boiler feed pumps.
Or look at some sort of Positive displacement pump ( piston, screw etc) where flowrange is more like 30-110% but pressure can be 0-150% of rated at vitually any flow.
Feed water inlet pressure is about 0.3 bar so is fairly negligible in the grand scheme of things.
you've chosen a very special name for a handle - I hope you're up to it... welcome to ET, just make sure you read my strapline.
LI
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Pumps
RE: Pumps
Our philosophy is that once the differential pressure across the pump is found we can then use the signal from the DP cell through a controller and a VFD to make the pump head to be always greater than the boiler operating pressure hence ensuring flow. We also have a flow monitor that monitors flow to make sure that flow is happening when it is supposed to happen.
LI we have boilers operating from 11bar-20bar. We would then want the DP cell to measure up to 30bar. We intent to mantain it at a fixed level. We also have two pumps running in parallel.
Kind Regards
IsaacNewton4
RE: Pumps
Kind Regards
IsaacNewton4
RE: Pumps
11 to 20 bar is a massive pressure range for parallel centrifugal pumps if you're not controlling them via a control valve.
How exactly are you thinking or doing the control "such that the head of the pump can be changed as the boiler pressure modulates." VFD? Control valve? different pumps?
If I was you I would just get pumps suitable for max pressure then install a control valve downstream to regulate flow and pressure accordingly. Simple, cheap, but you loose a bit of energy when operating at 11 bar.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Pumps
RE: Pumps
So you appear to have a system which works, but has some level of liquid level variation. how much level variation? does it cause issues with the boiler?
When you say in your post above "We intent to mantain it at a fixed level" - Intend to maintain what? the differential pressure or the liquid level?
Why 2 pumps in parallel?
also what is a "flow monitor?" A flow meter or a simple flow / no flow device?
You say you don't know how to proceed, but what don't you know? You seem to have a working system so what is the issue here?
This is why it's best to describe your system in the original post, preferably with a diagram or two ( can be sketches) and clearly state what the parameters are, what you've tried and what your issue is. Drip feeding information is frustrating for those of us who can't see your design...
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Pumps
RE: Pumps
Fix the VFD to a static high rpm suitable for 20 bar and then add a control valve controlling on level. Once you tune the PID controller it will be rock solid on level.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Pumps
RE: Pumps
The first error I made in explaining is that we have 11 bar boilers, 14 bar, 17 bar and 20 bar boilers, not that the pressure in the boiler is designed to vary from 11-20 bar. So we intent to keep the liquid level of water in the boiler constant. We use two pumps but only one will be running at a time the other is for standby. Then the flow monitor is a flow switch which detects whenever there is flow. The main issue we have now is that the water is oscillating about the required setpoint whereas my boss wants the water to remain solid level at the required setpoint.
Thanks
IsaacNewton4
RE: Pumps
Just set the DP cell to say 3 bar above boiler pressure and add a control valve ( 3 bar should be enough) and then control on level using the control valve. Once you sort out the PID loop control it will get you much much better control over your level control.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Pumps
RE: Pumps
-Blake
"If it rotates, it vibrates."