9 replies to this topic

[silkroute]

Dear All,

 

What should be a design pressure of particular line attached with equipment ? A design pressure of equipment or design pressure as per piping class ? 

 

I want to fill up line list. Could you please guide me?

 

silkroute..

[fallah]

silkroute,

 

The design pressure of the piping around an equipment up to first isolation valve in each line could be the same as that of the equipment.

[silkroute]

Fallah,

 

Is it first isolation valve or last isolation valve ? Is there any standard / guidelines which recommends first/last isolation valve?

 

silkroute.

[paulhorth]

Silkroute,

Allocating design pressures is an important part of process design. Your plant should be made up of systems of equipment and piping each with a common design pressure, protected by PSVs set at this pressure. Where these systems connect, there will be a design pressure break. You should identify the exact locations of these breaks on a set of P&IDs. The rule should be that a design pressure extends to the furthest isolation valve from the source of the pressure in each piece of connected piping. Note that a check valve can act as an isolation valve in this sense (trapping higher pressure).

You will need to study each PID in turn to determine where these design pressure breaks should be placed. This is your responsibility. Assume any isolation valve could be closed, or open, whichever leads to the furthest extent of the highest pressure.

When you have marked up a set of PIDs you can then mark up the line list to be consistent, and then allocate the piping specs.

 

Design pressures for piping are thus established by equipment design pressures, not piping class limits. Piping class limits will be higher, and therefore safe, but specifying these as design pressures will lead to unneccessarily high hydrotest pressures for the piping, and also will give rise to queries about seeming inconsistency between vessels and piping.

 

Paul

[silkroute]

Hi Paulhorth,

 

You are absolutely right but many times I observed that Instrument engineer buying his instrument based on design pressure/design temperature as per piping class, which is costly options e.g. if instrument engineer specifying design pressure/design temp as per piping class on actuated valve data sheet then vendor will design actuator based on full shut off pressure as per piping class which gonna be bigger in size obviously.

 

Similar thing with manual valve trim, if we consider design temperature as per piping class then selected trim material will be higher graded unnecessarily. why project  buying valves as per piping class ? Even though we don't ask vendor, they do pressure testing as per piping class regardless of process defined design pressure. 

 

silkroute

[fallah]

Fallah,

 

Is it first isolation valve or last isolation valve ? Is there any standard / guidelines which recommends first/last isolation valve?

 

silkroute,

 

First isolation valve starting from the equipment...

 

In rare case it might the design pressue of the equipment be higher than the associated piping, then the design pressure of those piping lines should be regarded as per relevant piping classes instead equipment design pressure...

Edited by fallah, 28 April 2013 - 10:06 AM.

[Dacs]

You are absolutely right but many times I observed that Instrument engineer buying his instrument based on design pressure/design temperature as per piping class, which is costly options e.g. if instrument engineer specifying design pressure/design temp as per piping class on actuated valve data sheet then vendor will design actuator based on full shut off pressure as per piping class which gonna be bigger in size obviously.

It's unnecessary (and costly) to size your actuator based on maximum pressure as per piping class.

 

Similar thing with manual valve trim, if we consider design temperature as per piping class then selected trim material will be higher graded unnecessarily. why project  buying valves as per piping class ? Even though we don't ask vendor, they do pressure testing as per piping class regardless of process defined design pressure.

This is just a matter of convenience since valves are manufactures as per certain standard rating (150#, 300#, etc). It's costly to ask the vendor for valves (for instance) that matches your project design pressures.

 

So normally you'd pick the minimum rating that covers the design pressure of your piping.

 

Just to add, design pressures in piping also covers other components of your system such as flanges/gaskets/o-rings/etc. So while you may have a 600# piping, your gasket may not work on the maximum pressure your 600# can handle.

[paulhorth]

All,

 

It appears to me that in Post no.6, Fallah is saying the complete opposite of what I said in Post no.4. This may be a misunderstanding by me. I am going to leave it there, because I will be away this week. The readers of the forum can decide for themselves whether the design pressure of a system should extend to the furthest isolation valve from the source of pressure, or only to the nearest.

 

Paul

Edited by paulhorth, 29 April 2013 - 02:21 AM.

[Dacs]

I think that boils down to the particular system in question. 

 

In closed drainage for instance where you'd have 1 valve near the vessel and another valve near the discharge manifold, I'd normally rate the nearest block valve the same as vessel pressure and the 2nd valve as whatever the design pressure of the closed drainage system for the reason that the nearest block valve is normally closed during normal operation and you'd normally open the 2nd valve first to drain the system.

 

When it comes to pump manifold however, I'd use shutoff pressure to rate the piping from the discharge up to all the way until the farthest block valve (also including the nearest block valve at the suction).

 

So I think it's part of our work to decide on these matters.

[fallah]

All,

 

It appears to me that in Post no.6, Fallah is saying the complete opposite of what I said in Post no.4. This may be a misunderstanding by me. I am going to leave it there, because I will be away this week. The readers of the forum can decide for themselves whether the design pressure of a system should extend to the furthest isolation valve from the source of pressure, or only to the nearest.

 

Paul

 

Paul,

 

The matter is misunderstood by you as you did mention. When i mentioned first isolation valve starting from the vessel i didn't regard the vessel as pressure source. In fact, in most cases that design pressure of the equipment is lower than that of the associated piping and the equipment, let say vessel, is to be protected by a safety device at relevant design pressure then the design pressure of the each piping line around the vessel up to the first isolation valve can be specified equal to vessel's design pressure. Your logic based on starting from pressure source is a right but general procedure and i limited my judgement to a vessel and piping around because i wanted to submit a specific answer to specific request of the post originator.