Design pressure & temperature for drains & air vents piping? 1

[laxmikant07]

Dear Sirs,

How we decided drain & air vent piping design pressure & temperature after 2nd isolation valve for boiler proper & non boiler external piping as per ASME B31.1?
As in my case we planned to collect all drains & vents in common drain & vent collectors. Please advice how to decide design parameters after 2nd isolation valve to common collectors.

 

[BigInch]

Open all isolation valves.

Independent events are seldomly independent.

 

[LittleInch]

You need to work out where your vent and drain system then has no chance of blockage or accidental valve isolation until it reaches the atmosphere. If there is any chance of this then your vent and drain pipework needs to be rated to the same as your vessel regardless of what isolation you have from your vessel / boiler. Assuming you can assure yourself that it won't see full vessel / boiler poressure then you need to work out the maximum pressure in the vent/drain assuming that you open all the isolation valves fully and the boiler is discharging at max rate to atmosphere via your vent OR drain. This can be quite a high pressure, but you cannot assume, in design, that the operator will only slowly open the valve or use the throttling valve correctly. I have often seen an RO in place to limit flow into a drain or vent for this exact purpose (avoid huge flows which overpressure the drain/vent), especially for gas and steam.

This is often a key item when you HAZOP the system and needs to be looked at carefully as early as posisble as the design pressure and temeprature of vents and drains is often very low compared to the boiler. Getting it wrong is frequently a cause of failure and incident when a vent or drain is over pressuriseid and ruptures or is exposed to much lower or higheer temperatures than design. Just becasue it's "the vent" or "the drain", doesn't mean you can ignore the potential for over pressure. Equally having to change it later in the design is also a real pain.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[laxmikant07]

Dear Sir,

Very very thanks.

As your reply, can i take drain & vent design pressure & temperature as vessel/main piping pressure & temperature.

Sir you have any ASME B.31.1 code reference for selecting these parameters.

 

[LittleInch]

Design pressure of the main vessel for vents and drains might be a conservative value, but yes you could. Temperature may be different, not on the max value so much, but depending on the product, venting especially could result in a very low design temperature much lower than the vessel due to gas cooling effect as it expands so this needs to be analysed. Steam less so, but need to do the pressure / flow calculation.

This is simply a matter of establishing your design parameters and looking at your system to see what design ratings are justified or not. If you have a free vent or drain pipe with no valves to atmosphere that can't get blocked then this can often be a lower pressure rating (sometimes much lower) than the vessel. It all depends on your design.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[laxmikant07]

Dear Sir,
Thanks for reply.

I am not reaching to final decision for selecting these parameters. i will give you one example as following. if possible you are requested to tell me final value as your consideration.

1. I take 1 drain from the main steam piping having design pressure 171.0 Kg/Sq.cm(g), temperature 550Deg.C. For drain piping up to 2nd isolation valve i keep full pressure & temperature. now i want to know how i decide these parameters after isolation valve to common drain collector. (this length nearer 50m in length).

2. In other case i given one vent in piping having design pressure & temperature as 171.0 Kg/sq.cm (g) & 353 Deg.C. so please suggest design parameters after isolation valve to common vent collectors.

thanks in advance.

 

[LittleInch]

I can't do this as a remote exercise without lots more data and then becomes more like work rather than a forum question. See my posts above, but you need to look at this with a process engineer and determine what flows and pressures the piping could see in the worst set of circumstances - usually someone opening the vent or drain fully whilst the vessel is at full pressure / temperature - and then calculating the flow and pressure in a program or by analysis.

Only you know the relative sizes of the drains, vents and header and other things venting into your common header. A vent and drain study when you have many items connected can be a major study to establish what the maximum pressure the system will see without being too conservative. I've not had direct experience of steam so cannot comment on what happens to the drain if you accidentally dump super heated water into it, but it sure isn't going to be pretty.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[laxmikant07]

Thanks Sir,

I need help from other forum member also if any one can do.

If any one have ASME B.31.1 code reference for selecting these parameters. please tell me.

 

[LittleInch]

If the common drain and vent collectors allay exist then as a default position just continue with the design temp and pressure to the connection. If it's relatively small bore pipe and only 50m you won't save much by reducing these parameters. You may need to check that the existing "collectors" can handle the extra flow if this is an addition.

It can look a little bit odd having a pressure break at at tee, but has happened in the past for this exact reason, i.e. pressure in a small drain line can be a lot bigger than in a much bigger header.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way