7 replies to this topic

[shekhar dhuri]

Hi,

Seek valuable opinion on Split range and Cascade Pressure controls.

Though mentioned in the literature regarding split range pressure controls, none of my vendors are providing split range pressure control for Slug Catcher, Amine Column, Stabiliser Column, De-ethaniser column, de-butaniser column. There is just a single PCV (pressure control valve) which controls operating pressure. However, PCV to flare is not provided.

In my previous projects, we used to have split range pressure control i.e. Single PIC (pressure controller) giving output to two PCVs. PCV on the main line will control operating pressure of the equipment and PCV on the flare line will relieve the gas to flare if pressure exceeds the above maximum operating pressure. Needless to say that, PCV to flare was not used to eliminate the requirement of PSVs.

I am also used to see Level-Flow cascade control instead of mere Level control for the equipment such as Slug Catcher, Amine Column, Stabiliser Column, De-ethaniser column, de-butaniser column. My understanding is all these equipment have some amount of liquid during normal operation of the plant. Without the flow control loop, the level control loop will continuously oscillate for a long (slow) period, which will quite likely affect the downstream process. With the fast flow control loop in place, the sticky control valve will oscillate, but at a much shorter (faster) period due to the inherent fast dynamic behaviour of a well-tuned flow loop. It is likely that the fast oscillations will be attenuated by the downstream process without having much of an adverse effect.

Can you please confirm my understanding specifically for applications such as Slug Catcher, Amine Column, Stabiliser Column, De-ethaniser column, De-butaniser column. None of the vendors seem to follow this understanding and we dont want to change the standard vendor designs.

Regards,
Shekhar

Edited by shekhar dhuri, 31 December 2013 - 01:58 AM.

[himant]

For the de-ethaniser, de-butanize and slug catcher, normal config is to provide PCV to flare to have stability of plant in case of any disturbance like loss of condenser duty, etc. Not sure about the stabilizer column. Whereas for amine column, there is no need to provide PCV for flare because chance of over-pressurization due to abnormalities in running plant is rare. (Blocked outlet is other case).

 

Also clarify about the vendors? Are they vendors of column only or whole technology? If they are vendors of column, they are not concerned if you provide PCV at outlet or not!

 

Regarding control philosophy:

If you choose to have flow control loop, system will work fine provided there is no flooding, weeping, foaming or other disturbances. Unfortunately, that is not the case. Hence, it is always preferable to provide level control loop instead of flow control loop.

 

Regards

Himant

[fallah]

Can you please confirm my understanding specifically for applications such as Slug Catcher, Amine Column, Stabiliser Column, De-ethaniser column, De-butaniser column. None of the vendors seem to follow this understanding and we dont want to change the standard vendor designs.

 

 

Shekhar,

 

Configuration and architecture of the system control or specifying the control philosophy for a process plant is always among the responsibilities of the relevant basic designer/Licensor not required equipment's vendor.

[fallah]

Hence, it is always preferable to provide level control loop instead of flow control loop.

 

 

 Himant,

 

OP meant level-flow cascade control instead only level control. Then he didn't mean flow control instead level control...

[fallah]

I am also used to see Level-Flow cascade control instead of mere Level control for the equipment such as Slug Catcher, Amine Column, Stabiliser Column, De-ethaniser column, de-butaniser column. My understanding is all these equipment have some amount of liquid during normal operation of the plant. Without the flow control loop, the level control loop will continuously oscillate for a long (slow) period, which will quite likely affect the downstream process. With the fast flow control loop in place, the sticky control valve will oscillate, but at a much shorter (faster) period due to the inherent fast dynamic behaviour of a well-tuned flow loop. It is likely that the fast oscillations will be attenuated by the downstream process without having much of an adverse effect.

 

 

Shekhar,

 

In the cases such as columns bottom line flow variations are very much unsteady due to level indicated might timely move up and down; then using a LC/LV configuration will result in unsteady flow. 

 

If LIC/FIC cascade control will be used instead; the response of control system to variations will be faster and the flow will be more steady than that of LC/LV system.

 

You can reflect your concerns in this regard to the plant designer if it's possible at the current stage of the project...

[shekhar dhuri]

Himant,

Design of Process controls is also responsibility of the Vendors. Since they are supplying skid based units (Inlet Separation, Sweetening Unit, Dehydration unit).

[Bobby Strain]

You get what you pay for.

 

Bobby

[paulhorth]

 

You get what you pay for.

 

Sometimes you get even less than that.

 

Shekhar,

In general, I agree with your comments on the control systems in your proposed plant.

Provision of split-range pressure controls with vents to flare, at locations with separated vapour streams, is standard practice. This function is not only to allow for upset conditions which generate high vapour flows (such as loss of condenser duty, excess light components etc) but also to allow for plant start-up, when you will need to flare off-spec gas from various places (such as downstream of the dehydration) until the plant is fully online.

Level control cascaded to flow control, on liquid outlets from columns etc, is good practice, but applying it would depend on the downstream process. If the liquid is just going to storage then straight level control will be OK in my view. Likewise the flow of rich amine from the contactor is probably OK just with level control, because the flow of the circulation is usually controlled elsewhere ( on the lean amine). 

 

In your review of bids, it looks like you will have some comments to your vendors on the control philosophy, along with those for the shutdown philosophy and isolation philosophy which you have raised in another post.

 

Paul