Control Valve for depressurisation. 2

[Chemitofreak]

Hi,

As per API RP 553 (Refinery Valves and Accessories for Control and Safety Instrumented System) Clause 10.2.1 Control valves maybe used for depressurising service.

I have never came across a control valve being used for depressurising service. I have seen a general arrangement of a BDV (Blow Down Valve) along with a RO (Restriction Orifice), with a minimum distance of 600mm or 1000 mm between them to avoid cold creep to the BDV.

Have any of you have used/seen a control valve for depressurising service?

If yes, then what is the criteria for the same.

Thanks in advance.





Dinesh S SHELATKAR.
Process Engineer

 

[TD2K]

If the blowdown rate sets the flare capacity or you need to restrict it to stay within the existing flare capacity than an orifice is common. You simply have much more control over the actual area since you can get a bore done to any size and therefore control the flow to a specific value.

A wide open control valve theoretically can be used but you have much less control over the Cv of the valve you can pick. They tend to be in discreet capacities with relatively large "jumps" between one Cv and the next. Not typically a problem for a control valve but for a depressuring system, that could be another matter between not meeting your blowdown time (too small a Cv) to overloading your flare (too large a Cv). Yes, you could go for a custom trim but an RO downstream of an on/off valve is likely going to be cheaper.

If I had to put in a blowdown system on a smaller unit or process where the flow didn't set the flare capacity, a control valve might be a good choice. I'm not sure it would be less expensive than an on/off valve with a downstream orifice as you describe and the on/off valve is quite likely going to be a tighter shut-off.

 

[georgeverghese]

A depressuring valve is a non code venting device, and as such, it is up to the plant designer to set up the BDV for the purpose intended.

In some companies, a fail close control valve with a mechanical max travel stop and a secure air bottle is used as a BDV. In others, it is a FO, full bore ball valve with RO. In some companies, operational depressuring may be done with FC valves, while emergency depressuring is to be configured with a FB FO ball valve with thick plate RO.

 

[Chemitofreak]

@ TD2K and George Verghese,

Thanks for a prompt reply.

Have you guys used a PCV for depressurising in any of your projects?

Can you site some name of the clients?

@ George Verghese,

Can a depressurising valve be of 'FC (Fail Close)' type?

What about its probability of failure on demand.



Dinesh S SHELATKAR.
Process Engineer

 

[e43u8]

George,

In some companies, a fail close control valve with a mechanical max travel stop and a secure air bottle is used as a BDV.[unquote]

A fail close pressure control valve with mechanical stop isn't considered as depressurizing device/BDV at all and even though conducts the HC gas to flare network; is to be considered, in addition to a BDV, mostly as pressure maintaining device...

 

[Chemitofreak]

@ e43u8

Completely agree with you.

It is pressure control rather than depressurising. We associate the word depressurising with relief during fire scenario and not during process upset.

Dinesh S SHELATKAR.
Process Engineer

 

[georgeverghese]

Pls note I didnt say "pressure" control valve - obviously a depressuring valve needs to be a dedicated CV. A pressure control valve cannot be used as a BDV. A BDV is not a PSV.

Pls note that a BDV is not a code safety device - it is only used to mitigate the risk of loss of containment, so it is not appropriate to look at PFD values for this function - in some companies, an FC valve is used for operational blowdowns, and these may even be operated on flow control. In other companies, I have seen them using FC valves with dedicated secure air bottle for emergency blowdown.

In the absence of plant owner engineering guidelines, it is your choice to engineer out this function and add on for additional features as you see fit. The intrumentation engineer also has a role in this. The most obvious safety concerns would be (a) whether this valve can overload the flare system and (b) will it / does it need to function in the absence of plant instrument air.

 

[e43u8]

George,

"Pls note I didnt say "pressure" control valve - obviously a depressuring valve needs to be a dedicated CV. A pressure control valve cannot be used as a BDV. A BDV is not a PSV."

With your description as: "...a fail close control valve with a mechanical max travel stop and a secure air bottle is used as a BDV..." it has to be imagined a pressure control valve not a level or flow control valve; because only a pressure control valve releases additional pressure and also is normally closed and opens just in the cases of process upsets...

in some companies, an FC valve is used for operational blowdowns, and these may even be operated on flow control. In other companies, I have seen them using FC valves with dedicated secure air bottle for emergency blowdown.


The term "control Valve" in emergency blowdown system just refers to on/off control valves such as BDVs. For operational blowdown it's not required using automatic flow control vlave and the job would be done manually through, let say, PSV by pass line along which there can normally be a manual globe valve for flow adjustment...Indeed, a FC valve isn't allowed to be used in emergency blowdown line even if being equipped with secure air bottle...

 

[don1980]

An emergency blowdown design is a supplementary layer of protection. It is not code mandated, nor is it a substitute for the relief device required by code. Since it's a supplementary layer of protection, one can use any type of valve for this purpose (FC control valve, FO control valve, on/off valve, or no valve at all). There are no mandatory requirements regarding the integrity/reliability of these designs. The design integrity is a risk-management decision for the equipment owner to make.

In designing an emergency blowdown system, one of the key design constraints is auto-refrigeration (embrittlement). This effectively limits the blowdown flowrate. refer to API 521 for engineering guidance for emergency blowdown design.

 

[e43u8]

An emergency blowdown system of a pressurized node/fire zone, if recognized to be considered, is normally implemented on relevant plant's ESD system; then the relevant BDV, if it's intended to be compatible with such configuration, should be an full/reduced bore on/off control valve with FO and TSO (tight shut off) characteristics; and yes one might use another valve with different characteristics such as FC modulating control valve in an emergency blowdown system but it would have major contradiction with inherent characteristic of ESD system which normally needs the valve as final element of the blowdown system to be FO...

 

[don1980]

@ e43u8 - I think it's important for engineers to distinguish between recommendations and requirements. They need to understand where they can apply discretionary risk management judgment, and where they have no such discretion. Knowing those boundaries is essential for regulatory compliance and for generating safe designs based on the specific needs of the specific case being studied.

You stated previously that "a FC valve isn't allowed to be used in emergency blowdown line even if being equipped with secure air bottle...". It's OK to caution the reader about using a FC valve but to say an FC valve "isn't allowed" is misleading and inaccurate.

The bottom line is that engineers should implement safety designs that safely manage the specific risks, and comply with regulatory requirements. To do this they need accurate info so they can make well-informed decisions.

 

[e43u8]

don1980,

For a specific matter, i.e. characteristics of a BDV as a part of emergency blowdown system connected to ESD, generally speaking can't help and clarify the matter at all; but what is obvious in all such emergency blowdown systems: FO characteristic for BDVs which need being de-energized to open is a requirement not a recommendation...

 

[MortenA]

If i understand you correctly you are thinking of the "type" of valve (ball, gate, globe etc) that can be used for a BDV valve.

While i would certainly say that a ball walve with a downstream RO is the most common i also believ that i have ssen control valve type valves used (globe i think). IMO that weakness here will be the tightness of the valve. While a ball valve (that is not used frequently) would stay tight globe valves would be more likely to start to leak. Considering that the inventory is vented and the risk of hydrate/ice formation i think this would be good reasons to avoid e.g. globe valves as BDV's

Bes regards, Morten

 

[Latexman]

What Regulation/Code makes FO characteristics of BDVs mandatory?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[e43u8]

Fail position of a control valve isn't a characteristic would need to be dictated by any code; and is normally specified by safety considerations. Hence a BDV and pump's minimum flow CV have FO and a SDV/ESDV have FC characteristic according to their relevant functions in plant's safety system...

 

[Latexman]

OK, that's what I thought. I'm not disagreeing, but I was getting a little confused by the vocabulary (word choices) in the thread. No problem!

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.