Low Pressure Boiler Feed System

[BronYrAur]

I have an existing low pressure boiler feed system that is becoming problematic (almost from the start). It is not even 1 year old and we have already lost 2 pump seals and one feed water solenoid.

The system consists of four (4) 4,000 MBH boilers, all fed from a single feed pump (with standby pump). Each boiler has a 2-position solenoid valve that cycles to maintain water level.

At first, the pump did not run continuously, but the demand was causing it to cycle every few seconds. So, we install a small bypass back to the feed tank and ran the pump continuously. This caused an increase in the water hammering effect to the point where we had to provide additional bracing on the feed line. Each boiler has a water hammer arrestor, but that doesn't seem to be helping much.

It has been suggested that this setup is poor and is probably the cause of the pump seal failures. It has also been suggested to install either slow acting solenoids or some other modulating control.

This fix will be an out-of-pocket correction, so I need to be economical. Any ideas on how to best deal with this problem?

 

[BigInch]

Running the pump continuously, but you didn't say at constant flow. What's causing the hammering is the question. Have you a fast acting control valve, is the pump overpowered or operating at favorable flowrates, is suction pressure dropping and recovering, is the pressure in the boilers changing radically due to load variations?

If you can give flowrates, pressures, line diameters and lengths, elevations, with valve type and locations, pump characteristics, etc. it might help in identifying some possible causes as to how the transients originate and the best way to calm them down.

http://virtualpipeline.spaces.msn.com

 

[RossABQ]

Solenoid valves are a poor choice for this service. You don't need fully modulating control but a real control valve with specific characteristics would help. I have never seen a direct-acting solenoid valve in this service, are you sure it isn't a solenoid in the air line to a diaphragm on a control valve?

 

[BronYrAur]

No, I'm quite sure they are electric solenoids on the feed water lines.

 

[RossABQ]

These are really 4,000,000 BTUH boilers, each? I think I will go back on my previous statement and say modulating control would be most appropriate. Those are big boilers, and I'd assume firetube? When you say low pressure, do you specifically mean < 15 psig MAWP, a Section IV Heating Boiler?

 

[BronYrAur]

Yes, they are 4000 MBH boilers. LES model HF-4000-SG. See

http://www.lesboilers.com/hf&hwboilers.htm

for more info.

 

[RossABQ]

How would you characterize your loads? Smooth, constant loads or highly variable (like with some process)? You can't go wrong with modulating control, but it would be some bucks to change. If you must live with on/off valves, looping the feedwater thru an end-of main back pressure regulator to keep the pump at minimum cooling flow or greater would be a good start. These must be very large solenoid valves, at least 3"? Any way to regulate their opening/closing speeds?

 

[BronYrAur]

They are actually 1-1/4" valves. and the load is very constant. All summer, we only ran 2 boilers. Now as the weather is getting colder, we are starting to get into the third.

 

[TBP]

I'm not seeing anything in particular that connects the dots between the solenoid valve problems with the pump seal issue. It's very possible that these 2 issues are unrelated.

 

[BigInch]

If you can give flowrates, pressures, line diameters and lengths, ...

No info = no solution

http://virtualpipeline.spaces.msn.com

 

[BronYrAur]

The four boilers are all in a row in close proximity to the pump. Pipe leaving pump is 2-1/2" and contains a triple duty valve. There is a 1/2" bypass line here that routes back to the pump suction line. The 2-1/2" runs up 10' and over another 10' to 1st boiler. 1-1/4" drop to boiler and header reduces to 2". 2nd boiler is less than 10'away, as are each of the other boilers. Header reduces to 1-1/2" and then to 1-1/4".

I just noticed a descrepancy on my pump. According to the curve, it is rated at 59 GPM @ 20' (my submittal says 20 psig, but it is written in by hand). The curve must be correct, so I probably have about 16 GPM @ 20 psig. My boilers are running at 11 psi, so I probably have enough flow.

 

[BigInch]

You're out of the operating range then.

Have a look at the third bubble from the left. At 16 GPM and 20 psig, I'd put you're operating point right smack in the middle of that third bubble.

http://img210.imageshack.us/img210/6060/pumpcurveopsandreliabilrq0.png

http://virtualpipeline.spaces.msn.com

 

[BigInch]

delete "point"

http://virtualpipeline.spaces.msn.com

 

[TBP]

I've never seen a steam boiler feedpump discharge line with a "triple duty valve" in it. Triple duty valves are used on closed loop hot water circ pump applications. This seems linke an odd application for this valve.

The recirc line from your feedpump should discharge to the feedwater tank, not the pump suction.

 

[RossABQ]

I don't believe a single pump feeding boilers this size in battery meets codes unless there is an automatic city water back-up source (with appropriate check valves/backflow preventers). Triple duty is unusual only in that it limits max flow; if a boiler is low on water, why limit flow?