PID/PWM question

[Narlin]

If this works, it will be because I have the ability to control pump speed and the steam injection can reach a point where it is full open nearly all the time. I did get a boiler size in kW and it calc out to an equilibrium at close to 3 gym. I.e. at 3 gym I can leave the steam on all the time.

Narlin

 

[Tinker]

I would look at using a heat ex changer for the steam & water. It would be much easier to control a constant flow of the desired temp.. I do not know where the water in the pipe is going but there are chemicals in the steam that are added at the boiler that could cause you problems as well. If this is a food product you can get in big trouble here. By using a heat ex changer you can very easily control the temp with a proportional valve at either the input or output to the heat ex changer.

 

[danw]

Well, I guess to meet the project scope you can claim you're using PID for varying the water flow rate to achieve a setpoint temperature using always-on steam. That could work.

 

[Peter Nachtwey]

If this works, it will be because I have the ability to control pump speed and the steam injection can reach a point where it is full open nearly all the time. I did get a boiler size in kW and it calc out to an equilibrium at close to 3 gym. I.e. at 3 gym I can leave the steam on all the time.

Narlin

Is there another steam valve that can be throttled back so that you can reach equilibrium at 1.5-2 gpm? You don't want to control at the limit of your pumps capacity. I think this is the way to go. Then you can control the temperature by controlling only the pump speed and the temperature should respond very quickly to the pump speed.

 

[Narlin]

The engineer tells me that it is OK to modulate the steam blocking valve with a 4 sec period. It will open and close fast enough and he isn't concerned about mean time to failure. The steam/water mixing occurs in a penberthy jet. He is going to put in an orifice to throttle the steam into a range where flow is about 1.5 gpm. He suggests that we start with PWM on the blocking valve and try to tune it so that indeed the valve is nearly or always open. He is also going to use an orifice to control backpressure. The hot water is being cooled post transit by a HEX and then going to waste.

 

[daba]

On a job I worked on, (which was the only time i've ever seen direct steam injection), the injection point was so noisy they had to have it lagged, and lagged again, and lagged again, to meet noise regs. It looked like an elephant with all those jackets on.

Anyway, for me, the best way to control this is with a modulating steam valve. PWM will always create high and low extremes which may not be tolerable.

Did I read that you want to heat the water to 125C ? How are you going to maintain enough pressure in the system to prevent flash-off ?

Is this a CIP set ?

 

[leitmotif]

First thought I have is 122 C = 251 F. You will need to esure water is pressurized to ensure no boiling. Would guess minimum 20 psi.
SECOND you will to watch pump suction to ensure NPSH is high enough to ensure water not flashing to steam and cavitating pump or worst case running with steam inside and burning out shaft packing.
THIRD you make no mention of what the steam temperature and or pressure is. ALSO what happens to steam temperature when you shut off flow and line goes dead?
FOURTH You make no mention of tolerance of heated water temperature.
FIFTH What are you trying to control - total heat delivered, temperature of delivered water or flow of water.
SIXTH steam injection can work BUT the whole system must be designed for steam system pressure. I would recommend a steam to water heat exchanger.
SEVEN You will need a relief on the system for WHEN the steam valve fails open. It must be able to relieve full steam pressure and flow.

IF you are varying flow and want to keep water at a temperature (122C) plus or minus tolerance (XX ??) I have serious doubts about your ability to to this with an open / shut valve. You could possibly do it with multiple valves in a stage type sequence and have some hope of success. Otherwise as Dan said get ready for wild swings in temperature especially so with direct injection.

I think you will have best results with a modulating steam valve AND a small bypass valve to ensure you do not flash off cold water when you allow steam flow.

I recommend you sit back and scratch your head on this one and go back do some calculations and some redesign. You really MUST know what you are doing with steam or someone is gonna get hurt.

TO paraphrase Mr Nachtway - you will have poor results trying to get control algorythms to overcome bad system design.

Dan Bentler

 

[Narlin]

I started this thread to get an understanding about using a simple on/off steam blocking valve with PWM instead of an I/P. With your help and some experimentation on my part, that has been successful. Using a 4 second period (initially I was way off – thinking 300 sec), I can modulate the valve and tune the PID to admit steam, control temperature, pressure, and flow. COOL (or not).
Although I never really said what this project was all about, I will say now that it is a biowaste kill system using steam injection inline (not batch). It is the 3rd such system that my tiny company has done and we are still tweaking the system for efficiency. In that regard, we have replaced two I/P valves. One with a steam blocking valve, and one with an orifice. By & by, these are not simple systems and I wouldn’t ever downplay the need for interlocks, alarms, and safety. I have not shared a P&ID but suffice it to say that there are 30 alarms and 12 actuated valves along with three VFDs and 5 pumps. To many people that is trivial, but then all I am doing is heating up water.
Anyway, this particular issue is settled and I want to thank everyone who contributed.

Thank you,
narlin

 

[leitmotif]

Narlin

it is amazing how a single line description of system can deliver so much information.

You are right this is more complicated than just heating water. It is quite similar to steam distillation of drinking water. Proper temperature and adequate retention time (exposure time?) are critical to ensure adequate kill.

Dan Bentler