steam heater regulation

[lorang]

hello
I'd send this thread in chemical process forum, but I'd not a lot of answer, so I try in this thread.

I'm working on the regulation of a steam heater. It's capacity is about 2000 kW. So there are a control valve at the inlet of the steam. It controls the temperature of water at the outlet by controlling the flow of steam. A steam trap is at the outlet.
The problem is that the power required varies a lot (0 to 100%). So, there is a risk of stall of the heater.
The condensate line can be at 2 bara. We don't want to put a pump trap for cost reasons and because there are not a lot of place under the heater.
So, to eliminate the problem of hammer in the heater when there is a demand of power, I thing to put a deltaP gauge on the trap. When the temperature controller send a signal to open the control valve, this signal will be limited by a control of deltaP on the trap during of a period T. So at the start, there will be only a few amount of steam, that will limit hammer.
We are agree that there will be a bad control of temperature when the power is low, but this is not a real problem.
What do you think about this solution?
Thank you for your advise.

 

[TBP]

If I understand correctly, the end result of what you're proposing will limit the heat exchangers rate of response to a rapid load increase. What about just limiting the spike in the water flow at the point of use? Consider doing something there to ensure a more gradual load increase on the heat exchanger. This might be as simple as getting rid of a solenoid valve in the water line, and replacing it with a pneumatically actuated ball valve that can be set to open gradually. If there's multiple points of use that all come on at once, see if they can be staggered.

 

[lorang]

We can't change the load increase. We have put a water tank after it with a circulation to reduce the load increase. But with 2MW, with no load, it's look like nothing. The increase of load is the start of machines which must have a lot of hot water. So, when the machine is off, no load, when the machine is on, big load. No possibility to adjust it.

 

[Drazen]

This really depends on your load pattern.

If heated water is actually domestic (sanitary)water you can have relly extreme differences in demand - most of time one or two taps open and full load during shift change and use of showers and similar.

Normal way is to use accumalation tank and to size heater to be able to heat that tank in reasonable amount of time (which will you access - it can be half an hour, an hour or more).

What you probably have is oversized heater that can sustain full load during maximum use, but the rest of the time is exposed to severe water hammers.

The only effective way that I know for this situation is to separate flow in a heater, if possible, in several branches and put flow control valve in every branch. Every flow valve should have a bit different temperature setting so they will all open during peak period only, and only one will open during minimum consumption.

This is possible only if you have some kind of multi-coil heater where you can put valve on every branch.

If not, you need to install accumalation tank, the larger the better, depending on how much money you have.

 

[Drazen]

Sorry, somehow I didn't note your second post...
If your problem is big demand during work of your big machine (what kind of machine is it?) then machine working cycle seems as biggest problem. If machine is working continiuosly (i.e. it is on all the time), then you can arrange pre-heating cycle , an addition to machine commands that will not allow machine to start until water is heated, so it can be heated gradually.
If you have frequent on-off situation during normal run of machine, then you need to arrange some kind of accumulation tank that will accomodiate rush start up. You need two pumps: one which circulates water through heater and back to accumulation tank, the other which circulates water through machine and again back to the tank. You need vertical tank where you put machine-pump connections near the top of the tank, and heater-pump connections near the bottom, so laminated flow will exist between these two sections all the time.

 

[MintJulep]

Water hammer is most likely caused by condensation in the steam supply line when the valve is closed. When the valve opens, wham, slug of water gets accelerated into the heat exchanger. You could:

Modify the steam supply line to allow condensate to drain back to the boiler.

Add a separator/steam trap upstream of the control valve.

Install a bypass/bleed line.

 

[JimCasey]

Leslie Controls makes a range of self-controlled steam-heated water heaters, The control unit is feedforward, and integral. Instant response and no overshoot/undershoot. You could either get one of their units or understand their operating principle and try to emulate that with your equipment.

 

[rmw]

So does Graham Manufacturing.

And, I agree with JimCasey with regards to looking at how Leslie and Graham do this so successfully, and taking your approach from their methods.

I have used both, and they work well.

rmw

 

[abeltio]

see thread:

thread378-116256

does this thread discuss your case?



saludos.
a.