Installing a Ball Valve Downstream from a Safety Valve

A formal HazOp [Hazard and Operability] Study, a technique now widespread within the process industries, would throw out the posted suggestion on safety grounds.

The purpose of a pressure safety valve [PSV] is to vent overpressure in the vessel or pipeline it is intended to protect. Installing a ball valve, being an on/off device, downstream of the PSV in the vent line is therefore inappropriate at least, and potentially dangerous in the extreme, unless other protective devices, like bursting discs for example, are present in the plant item requiring overpressure protection.

Rhetorically, why would one want to do what is suggested in this post? If a ball valve is required in the line in question, then it would be better to put the pressure safety valve on a different branch with its own uninterrupted vent line so that both valves can carry out their intended function without the one interacting with the other.

This is what I was talking about...

check out this piece of PID, they say that ball valve shouldn't be there.

'They' might be right. Note that the valves upstream of the PSV are labelled, LO, standing for "locked open". Now, the original posted question makes no mention of an LO criterion for the valve downstream of the PSV! It might be OK for the downstream valve to remain if it too is LO except for maintenance purposes, and the lettering on the posted image is difficult to determine, so the remaining question is really quite simple: what does the HazOp Study report say about the valve downstream of the PSV (rhetorical questions)? Does it call for the valve in question to be 'locked open'? Are the keys for the lock under tight control?

It's time to check!

A valve of any kind (locked open or not) is a complete mistake. Any competent insurance company inspector would immediately demand that it be removed. Any competent professional engineer would refuse to approve such an installation.

Any inexperienced operator could inadvertently unlock that valve and close it.

Very true because the potential hazards and there is no reason to do it

The use of valves to isolate a PSV IS a standard. In order to do PMs on PSVs they need to be ISOLATED. The valves DO need to be CAR SEALED OPEN and documented on a CAR SEAL LOG. This is part of being a PSM facility.

PSM = Process Safety Management.

And YES the insurance company's like to do business with company's that have PSM facility's. In order to have a PSM facility, the management has to prove that they have procedures in-place to control and manage the facility.

On your P&ID it shows two FB LO valves upstream of the PSV. They are Full Bore Locked Open valves. also the ball valve in question is also labeled FBLO.

Further down on the P&ID it shows a PSZ-012B flowing into the same header.

It appears the there may be TWO PSVs on the same loop, allowing one to be taken out of service at a time for PM work. Also the PSV are also acting as an ESD depressurising valve. by isolating each valve, the function of the ESD coil and PSV can be tested without taking the plant down and costing $$$$!

Not only is it common, but it makes Good Sense to use a Ball Valve both up and down stream of a PSV.

To add some points on the installation arrangement of PSVs:

(1) When you have PSV (one or more) connected to a common flare header i.e. closed discharge system,the valves downstream of PSVs can not be avoided. If a PSV is discharging directly to atmoshere (at safe location) then there will be no valve on the discharge.

(2) Normally a Gate valve of pipe size (if PSV outlet size is smaller than pipe size) is preferred compared to Ball valve. The reason is that due to sudden discharge from PSV, there could be an incident of ball valve seals may get blown off. The Gate valve stem is pointing downwards or inclined below horizontal so as to avoid blocking the valve in case the gate and stem connection comes off in service.

(3) In order to avoid the human error of not keeping the inlet/discharge valve fully open, the valves could be LO with car seal arrangemnt as explained in the earlier response. In case of very critical application one can have interlocking key system (this is for more than one PSV in parallel) -- this will ensure that atleast one set of valve -- inlet and outlet will always be in open position.

Hope this answers the query.

The LO valve may be required for repair purposes without closing down the rest of the system which may even be more harmful.

Quite.

It all depends on the HazOp Study report. It's still time to check...

"it's not appropriate to install a ball valve at the downstream"

No. You can install .......................

You can install the appropriate valve at either upstream and/or downstream, depend on your need for that valve.

Suppose you have a flare header connected to all downstreams of all safety valve at certain plant, and suppose that such a repair or maintenance is required to the equipment in which that safety is connected. To do that maintenance you have to shutdown the equipment which protected by that valve and then close the downstream valve as an isolating valve. At that moment you can made any repair or maintenance including the inspection & repair for safety valve itself, and that avoiding shutdown of whole plant.

Only you have to lock the valves and only allow the authorized persons to have the key to ovoid off the valve while working.

Also the valves shall have a mark showing if it is on or off.

If you feel the need for a Ball Valve, remove the Relief, or Safety valve.

The two should not co-exist on the same line.

You need to consider the governing "standards" (if any) for the process that the underwriters would expect to be adhered to. API, ASME, ISO? They will spell out what is permissible.

The most common and accepted method for preventing process downtime due to a faulty SRV is to have a Y-pattern switching valve upstream of two SRV's. They must provide a constant flow area while switching.

Isolation valves are sometimes permitted but only if there are additional redundant failsafe devices. Rupture discs installed upstream of the SRV as an example. The disc is set at the MAWP and the SRV somewhat below. In this scenario the disc is the primary "code" safety device and the SRV is the secondary, used only to prevent the disc from bursting during transients. But again, the standards will dictate what is permissible.

You’re right - it’s generally not recommended to install ball valves directly downstream of a pressure safety valve. The reason is that a safety valve is meant to open fully to relieve pressure instantly. If a ball valve is closed or partly closed downstream, it can create backpressure or restrict flow, which can interfere with the safety valve’s operation and even damage the system.

Basically, safety valves work best with a clear, unrestricted path downstream - any isolation valves should be positioned where they don’t compromise that.