Dear all

My self Rahul, and currently working as process engineer with engineering consultancy firm.
During commissioning of one of the plant we face the peculiar problem regarding steam depressurizing and de superheating

Steam at 3.2 barg to be reduced to 1.5 barg with the help of pressure reducing valve and addition of DM water to de superheat the steam. We install the line with the pressure reducing control valve followed by spray nozzle to de superheat the steam. But after addition of DM water still we are not getting the required temp of the steam (i. e. saturated steam at 1.5 barg -127 deg C). Approx temperature at the down steam is about 145 deg C. Schematic showing the details of control loop are attached with this. DM Water temperature Control Valve is sized for flow rate of 3.5 m3/h Max. Stechometric Requirement of DM Water is just 460 kg/h but after opening the control valve on DM water line full open (manually) we are not able to achieve the required temp.

What’s gone wrong with the design? Please Advice.

Thanks in advance.

Rahul,
I did not go into detail to make calculation to check your data and results. I assume those data that you have given us are correct and reliable. Also I assume all instrument/piping run/design are correctly specified and designed.

MY quick sense tell me :

i) problem DM water supply line
ii) under-perform mixer / atomizer / desuperheater
iii) 10 m straight run is insufficient
iv) problem with TIC loop (TT, signal, cable, CV, etc)

A few point you may look at :

i) check the performance of the TIC loop
ii) check the performance of temperature transmitter and control valve (at least the DM CV) and recalibrate if required.
iii) check availability of DM supply (correct pressure level, flow, etc)
iv) check if the 10M run is straight run against design
v) IN equal OUT check if the injected DM is "bypassed" without well mix with steam and lost in the steam trap line

There are many things you can do and information to be provided so that we can help.

Looks forward your positive response.

Rahul,
One more important thing...

The DW water pressure is 3 bar with the desuperheated steam at 1.5 barg.

Is the "3 bar" mean 3 barg or 3 bar abs ?

Anyway... the alowable pressure drop is only 1.5 bar between the DW source to desuperheated steam user.

This 1.5 bar required for control valve, line, fitting, desuperheater instrument, etc. I really doubt if this is sufficient. the downside is reduction in DW flow and subsequently lead to high temperature on desuperheated steam.

Thanks for replying,

You pointed out in your first reply regarding malfunctioning of the temperature control loop, poor Supply of DM water (due to insufficient pressure), and position of steam trap from the spray nozzle or water by pass at spray nozzle without mixing. As per our on site inspection, TIC loop and DM water supply is not the problem but as you said possibility of water bypass without mixing with the steam and getting drain through steam trap may be the reason. Also it may be possible that the spray nozzle installed on the line is not forming the proper mist spray (which is helpful for evaporating DM water). In such case what would be the remedy to get the mist spray? Marginal increasing in supply pressure of DM water will serve the purpose or we need some further modifications.

At least some good news.

Please go back to the initial design intend.

Check
i) is the 10 m is sufficient for "miximg" ?
ii) Check if the injection/spray nozzle is provided as specifed.
iii) cehck the pressure drop profile for the DW line. I have the strong feeling that the DW supply pressure is insufficient.

Awaiting your positive response.