Hi friends, I have a temperature co 2

[chela]

Hi friends, I have a temperature control problem. We produce SHP (128Kg/cm2 and 540 Deg C) steam in our Ethylene Cracker heaters in the transfer line exchangers ( for quenching the cracking reaction). As the material of construction has a limit of 545Deg C, the trip setting for the heater is set at 540Deg C( SHP Temp). Our operators are maintaining the set point at 525Deg C, to avoid the trip. Occasionally, as the SHP header pressure swings up, the steam flow comes down and temperature shoots up and remains above 540Deg C for quite some time, causing the operator to keep the setpoint down. The material of construction is exotic and cannot be changed economically. Is there a way to get closer to 540Deg C, so that the turbine drives can get high temp SHP, which is has a higher sp. enthalpy and gives a higher BHP. Presently, our compressor drives and hence plant capacity are limited by the turbine steam flow, which is high due to the lower temperature of the drive steam (SHP). The temperature of SHP is controlled by a desuperheater in between the two superheater coils. We have tried tuning it many times but still could not lick the problem. Can anybody help?

 

[hacksaw]

You are set up for constant steam demand trough your SH. Metal temperature sets the min. flow requirement of the equipment.

On loss of the turbine load, you'll have to vent excess steam or dump to lower pressure headers. It is a pricey solution, but the only one that will handle upsets.

If by desuperheating you are indicating steam bypass valves, they can only trim the duty between the two SH sections. They cannot be expected to handle large flow changes and hold your metal temperature requirements simultaneously.

 

[chela]

Thanks hacksaw for the fast and right response. The Desuperheater is a typical atemperator, where BFW is injected into the Superheated steam line going to the second stage ( High temperature)superheater coil. the BFW flow is controlled by a TIC getting impulse from the final SHP temperature. You are 100%right that the solution of venting or depressurising to a lower pr header ( which again will ultimately vent to atmosphere ) is the perfect solution. There is already a pressure control valve venting to atmosphere from the SHP header but it is not able to handle the flow swing which is not directly proportional to the pressure but which un-fortunately directly affects the temperature of SHP steam. In other words, I am asking "Is there some special manipulated variable that can control the pressure of the SHP header in case of small disturbances by venting to atmosphere but act proportional to the actual flow reduction in case of severe pressure/flow disturbances?" OR can the steam flow somehow be linked to the pressure controller to do the above? If you will not be offended, May I request you to pl ask some of your known Instrument/Controls Tip expert to respond. Thanks in anticipation.

 

[hacksaw]

Chela,

You can decouple the the turbine from SHP header pressure somewhat by using "back pressure control". It only gives you a few kg/cm2 swing allowance, but it is enough to prevent the system from spiraling into an unstable conditions. Typically the valve cannot shut off against full header pressure so you must incorporate protection logic to prevent that from occurring.

We did not use attemperators because it worsens the control problem. The turbine draws what steam it needs to meet the load requirement. A heat duty increase from the process side will cause the attemperator flow to pick up to hold steam temp. Now you have excess steam with rising temperature with no place for it to go. Basically an unstable system.

We used SH series and SH bypass valves with a complex arrangement of controllers to maximize heat pick-up from the process stream (subject to temperature constraints) while stabilizing the SHP temperature (also subject to temperature constraints).

The use of SH series and bypass valve arrangement allowed for a stable pressure condition at the turbine inlet.

The system was stable in normal operation, minimizing the use of blow-off steam and maximizing available superheat for driving the turbine.

The most difficult design hurdle was coming up with a protection scheme on loss of steam flow that protected the superheater equipment.

The control scheme is not just the result of one persons input. It took two process engineers, two mechanical and a couple of controls types along with the piping design team to come up with a design that kept all of the components within their operating capability. It is not something that can be resolved in a comment or two.

Good luck, glad to be of help

 

[chela]

Thanks. If it does not voilate yr company policy, Can you share the control scheme with me ( My e-mail address is :krs_mani77@yahoo.com. thanks in advance