Which burst disk for LP tank with "+" rating?

[Tiny Bubbles]

Which burst disk should go on a low pressure tank with the "+" rating (2640 psi)? Should it get the 4000 psi burst disk like normal 2400 psi LP tanks or the 5000 psi disk like tanks that are filled to 3000 psi? Do they make an 'in-between' disk for LP+ tanks?

-Tiny-B.

 

[pescador775]

The correct disk is 4000 psi.

 

[DA Aquamaster]

The plus rating makes no difference as the burst disc is predicated on service pressure, not service pressure plus 10%.

 

[flounderboy]

also depends on how much you fill them

I never go over the rating

but use 5000 disks

dive safe

mark s

 

[Tiny Bubbles]

Thanks, all. I'm getting my first LP+ tank soon (an OMS 125 - comes without valve) and it's just one of those things I've never thought about until now. So the correct disk is 4000 psi.

-Tiny-B.

 

[FredT]

Actually the burst disk requirement in the United States is based on HYDRO pressure. US DOT currently requires disk failure at or before hydro with a -10% tolerance.
i.e. a 3000 psi Al 80 hydros at 5000 psi. The burst disk installed has to be designed to fail between 4500psi and 5000psi.

Newer "exception" tanks, especially the E-x-zzz series PSTs and the euro tanks being "brought into the fold" by proposed new regulations use a slightly different ratio of hydro to working pressures than the 5/3*working pressure formula common to the 3AL and 3AA tanks. In all cases the burst disk rating is off the HYDRO pressure.

FT

 

[DA Aquamaster]

Well.. you could look at it either way. The hydro pressure is based on the service pressure (regardless of the ratio used). So from that perspective, you can't determine the hydro pressure until you establish the service pressure.

Given the customary 5/3 ratio, for example, you start with the service pressure to determine hydro pressure rather than working the problem backwards using hydro pressure to determine service pressure on the tank. You work the problem the same way when figuring the appropriate disc to be used.