Requirements to "Isolate" a Branch from Main Line

[rconnor]

Hi Everyone,

I'm looking into adding a branch off of the blowdown line of a boiler that would house a pump and other equipment (check valve, gate valves, etc..). The branch would be used to mix the water in the boiler from the front blowoff drain to the back blowoff drain during wet lay up periods. Therefore, the pump would only be exposed to water at lay up conditions (room temperature/pressure).

However, when I was looking through B31.1 it appears that the pump needs to withstand boiler pressure/temperature and treat it like I would the actual blowdown line. The rise in cost from having a pump be able to with stand these conditions goes from $700 to $3500 and requires custom seals which greatly increases the delivery time.

Is there a way to, with respect to the ASME/CSA, "isolate" the line so I don't need to spec the pump to boiler temp/pressure?

Currently, my design consists of lateral branches off the blowdown line that form a U-shaped branch with slow opening blowdown valves on the entrance and exit to the branch and a check valve on the downstream side of the pump (all rated to blowdown pressure/temp). What additional equipment/valves would be required to "isolate" the pump from normal blowdown conditions. Again, the valves to this branch would only be opened when the boiler is in wet lay up (at near room temp/pressure) and so the pump would only be used under these conditions.

Thanks for any help/advice you can provide!

Ryan

 

[KoachCSR]

What code year of B31.1 are you building to?

I don't currently have a copy of ASME B31.1 to reference, but I think all you need is a pressure-reducing device (i.e. valve) at the inlet and outlet with a relief valve for each capable of passing the entire capacity of your branch line for overpressure protection. The relief valve would be downstream of the valve at the inlet and upstream of the valve at the outlet.

Note that the pressure-reducing devices would need to be designed for the parameters of the boiler, not your branch line.

At the worst, you may need to incorporate double isolation (again, don't have B31.1 in front of me at the moment). The key is that each relief valve (at the inlet and outlet) would need to have a capacity capable of discharging the full flow of the line at the worst-case overpressure condition. Depending on the flows here, it may be more economical to actually design the branch for the higher parameters as the size and/or number of relief valves may be prohibitive.

Perhaps someone with immediate access to the code could confirm or correct me?

 

[KoachCSR]

Allow me to clarify:

The pressure-reducing valve would serve to reduce the pressure if the boiler were active or during some transient phase. I assume this valve would be normally shut when your branch line is not in use, of course. I used the term "pressure-reducing device" because I beleive that is the language used in B31.1 as I recall.

Sorry for any confusion.

 

[rconnor]

Thanks for the input,

I'm building to B31.1 - 2007, and I will have to go through it again myself. As the first couple times I went through, I wasn't exactly worrying about isolating the line (but after I got the pump quotes back and saw it was 5x the price I was).

I was worried it would require a PRV, I thought if it only required a double shut off valve or something that I could save money on the less beefy pump vs a couple more valves.

I will need to look into this more myself but if anyone else has any input that would be greatly appreciated.

Thanks again KoachCSR.

 

[rconnor]

I've looked over B31.1 again and had a couple more things to add:

- The issue with the pump is the temperature not the pressure. The pump can handle the boiler operating pressure but the seal of the pump can't handle the boiler temperature

- B31.1 doesn't seem to discuss how to effectively isolate temperature conditions, it only talks about using PRV to isolate pressure conditions. Does this mean that a simple valve would do the trick to isolate the temperature?


Again, the pump and branch will never be opened when the boiler is at operating conditions, it will only be ran during boiler lay-up (when at room temp). But my concern is that according to B31.1 I have to spec the equipment in the branch to the operating conditions of the boiler unless I install some specific equipment/arrangment. I'm trying to figure out what that equipment/arrangment might be.

 

[metengr]

rconnor;
I think you have flexibility in selecting your design conditions for the blow-off piping provided it is after double isolation valves from the main blow-off piping header.

Read 122.1.1 (B) specifically.

 

[metengr]

...also forgot to add, specifically review 122.1.6.

 

[unclesyd]

We have some similar systems where we use the double block and bleed system approach on several HRSG's. This piping is under both 31.1 and 31.3.