Set point greater than MAWP intermittent operations

[sandnes]

I am working on hydraulic oil system for life boat winch operations. The hydrulic oil unit will be operted once in a year for 2-3 hours for testing purposes otherwise it will remain closed until unles in emergency to lift the boat at the platform from sea.

The maximum operating pressure will be 210barg which is needed only once in four year for system certification purpose only.The available piping has design pressure (MAWP) is 220 barg. The piping code is ASME B 31.3 which allows to have accumulation of 20% over pressure for 50 hours at once and 500 hrs per year.

I have two options,
1)I am considering to put the PSV at 230 barg. With 10 % accumulation of PSV the maximum relieving pressure will go upto 253 barg which is 15% over pressure accumulation.

2) Set the PSV set point at 220 barg but this will make set point of PSV very close to max operating pressure and it may cause leakage.

I was wondering that is it safe design. But keeping in view the only once a year for short period of time first option is better.

Need suggestions.

 

[don1980]

Since we're talking about piping (B31.3) rather than ASME Sec VIII you have more design flexibility. You're not limited to the two option that you listed above, and based on my very limited understanding of this design I don't think either one is a good choice. Mainly, that's because this is a liquid-filled system (incompressible). Such systems generally experience pressure spikes that can occur far faster than what you'd experience in a gas/vapor service. That means that when PSVs are used in these systems, you need a very wide operating margin between system operating pressure and PSV set pressure. Otherwise, you should expect to have frequent lifts of the PSV.

I think a far better solution is to use a pressure limiting bypass device, such as a Fulflo valve. Those valves are designed for this type service (hydraulic systems). They're like a PSV except they have far better modulation characteristices. You'll find these on almost all lube oil skid units, on the discharge of the pump.

This is a bit of a unique application because you're designing overpressure protection for a critical safety system rather than for process equipment. The first priority is to ensure that the design works. In other words, be careful to ensure that the overpressure design doesn't impair the normal operation of the equipment (rescue people from life boats). If 210 barg is necessary to save someone's life, then we have to be sure the PSV protect the mechanical system and prevents the system pressure from dropping below 210 barg. A liquid service PSV requires much more blowdown than a vapor service PSV. A liquid service PSV design should allow at least 15% blowdown. In some cases the blowdown might be ~20%. Since this is a life-critical application, I'd add a generous safety fact to that just to be sure the PSV didn't get in the way of rescuing someone. This means the PSV set pressure must be much higher than the values you listed in the two options. Even better, I'd design the system such that the PSV wasn't necessary - again, use a pressure limiting bypass valve instead.

 

[sandnes]

Thanks Don for quick reply.

The operating pressure during emergency or boat lifting is 150-180 barg and pump has internal bypass relief which sets at 210 barg. This pump internal relief start bypass if pressure increases more than 210barg. I think it is the same as you are talking about i.e fulflo bypass valve but it is built in within submerged pump and tank unit.

The high pressure 210 barg needed only during certification because it will be performed with 1.5 more load than empty boat.

there is already one barrier i.e pump internal rbypass elief sets at 210 barg but i think it is not considered a proper PSV and its difficult to get its maintenance as it is built in.

Do we have any regulation saying that bypass valve is considered as proper PSV which is maintainable?

If the bypass relief valve is considered as proper PSV then maybe we dnt need any further PSV. But if it is not considered as PSV then we can install the additional protection at 220 barg as normal operating pressure will be 150-180 barg.

Any comments!

 

[don1980]

Unlike pressure vessels, relief devices are not required for all piping. Relatively few segments of piping have a relief device. That's because most piping is designed for the highest pressure that can be developed. When you install a pressure limiting bypass valve (e.g. Fulflo) and set it at a pressure that's below the limit of the piping, then you've satisfied the phrase "...designed for the highest pressure that can be developed".

I'm assuming we're talking about a PD pump. Most PD pumps come with an internal pressure relief valve. If they're periodically tested, then yes they can be relied on for protection. But, since these are internal to the pump (and not off-the-shelf valves), I rarely hear of anyone who includes them in their routine PM program. You can't assign integrity to anything that's not periodically tested/inspected.