4 replies to this topic

[Matteo Giorgio Marrano]

Goodmorning,
i have a desalter plant consisting into two stage+recycle water.
First stage desalter Pressure: 8.6 barg
Second stage desalter Pressure: 7.2 barg
PSV set pressure: 14 barg

My question is simple: considering to put on the 2nd stage desalter vessel a pressure switch PSH who gives me an alarm to DCS, what should be the set of this Pressure Switch?SOmething less than the 14 barg of the PSV set?

Another question: the PSH gives an alarm to DCS, and then?It stops there with an alarm to DCS or it goes to shut down the plant?What is the function of a PSH mounted on the vessel like in this case?

I hope my data are sufficient for the answer..

thanks

Matteo

[Dacs]

You have to first consider the ultimate purpose of a pressure switch that you're planning to install.

Alarms are for the benefit of the operators that run the plant. What benefit (as far as operation is concerned) will a PSH set to set pressure bring? Will an alarm set lower than set pressure give an operator enough time to alleviate the increase in pressure?

PSH by itself won't do anything. It must be interfaced with some sort of interlock to do something. Consider this however, shutting down a plant will cost you lost income, big time. As much as possible, you'd rather produce an off-spec product than shutting it totally.

[Whit123]

The set value of the PSH must be such that in case of a process upset (say a rise in the pressure of the desalter), it will give enough time to the operator to take corrective action. So you must estimate the rate of pressure rise that would happen and the time the operator gets to take action to reduce/vent this pressure without activating the relief valve (which will open at 14 barg). In case the relief valve opens, it will release the vessel contents to the surroundings. This would mean a loss of material as well as potential for injuries/damage- will the installation of the PSH prevent this?
Also consider the reliability of the pressure switch, would the impulse line have deposition of salt on it? In that case the line size has to be such that the material buildup will not affect the performance. Also in case the switch is required to be very reliable, you may consider having a pressure transmitter instead-so that you get a continuous trend of the pressure instead of just one point of high pressure after which everything happens very fast.

[fallah]

The set value of the PSH must be such that in case of a process upset (say a rise in the pressure of the desalter), it will give enough time to the operator to take corrective action. So you must estimate the rate of pressure rise that would happen and the time the operator gets to take action to reduce/vent this pressure without activating the relief valve (which will open at 14 barg). In case the relief valve opens, it will release the vessel contents to the surroundings. This would mean a loss of material as well as potential for injuries/damage- will the installation of the PSH prevent this?
Also consider the reliability of the pressure switch, would the impulse line have deposition of salt on it? In that case the line size has to be such that the material buildup will not affect the performance. Also in case the switch is required to be very reliable, you may consider having a pressure transmitter instead-so that you get a continuous trend of the pressure instead of just one point of high pressure after which everything happens very fast.

Youn mean PSH without interlock for required action and only with alarm?

[Zauberberg]

Usually, there should be a dedicated pressure controller which regulates the desalter pressure. Having said that, the purpose of pressure switch is not clear; the pressure controller itself should have a high alarm and one doesn't need a separate pressure switch just for providing a high alarm.

I don't believe you want to shutdown the plant in case of high desalter pressure - I have never seen such interlock. You could put the downstream crude furnace in danger by doing that. Only in the case the problem persists, you may consider shutting down the crude charge pump manually (from DCS) and proceed into regular plant shutdown.