Watch Out With Piping, Valves and Hoses

Consider common dangers during process hazard analysis.

By Ian Sutton, AMEC Paragon

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The PHA team should check the pressure rating for the overhead pipe and confirm its supports can handle a liquid-full situation. If they can’t, you must take special precautions to ensure the column can’t overflow. Also, a procedure for safely pressure testing the system with gas has to be developed and followed.

Underground piping. Such lines can be hazardous because not much may be known about their condition — “out of sight, out of mind.” Unusually high corrosion rates may afflict piping if a cathodic protection system wasn’t installed. Contamination of ground water or some other pollution event may provide first indication of a problem.

PHA team members should satisfy themselves that underground piping is properly inspected and maintained.

Hoses often are involved in accidents because, in almost all cases, they handle short-term or temporary duties. Sometimes, the service is routine — truck loading and unloading. In other situations, plants rely on hoses for temporary operations, such as to bypass a leaking valve that’s to be removed for maintenance. Whatever the reason for a hose’s use, there’s potential to release hazardous chemicals, particularly at the start and finish of the operation.

You should consider a number of situations:

Hoses and truck pull-away. If a truck connected to a process tank for loading or unloading drives away before disconnecting its hose, chemicals could be released from the tank, from the hose itself or from the truck. Safeguards to consider include:

• secondary containment around the loading station;
• intentional weak spots in the loading system that will direct any spill to a safe location;
• excess-flow valves on the truck and tank; and
• special operating procedures, such as removing keys from the truck until the operator has checked that the vehicle is safe to move.

Hose run over. If another vehicle runs over a hose that’s in use, the hose may split. Or it may get pinched, causing low flow or high pressure in other sections of the process.

Hose failure. Frequent use of hose can lead to flexing and abrasion and eventual failure. So, hoses require careful inspection and maintenance — and prompt replacement when necessary. The team should check that hoses to be stored outside couldn’t be damaged by water, freezing or sunlight.

Backflow prevention. When utility hoses are connected to a process, it’s particularly important to make sure that a backflow preventer (check valve) is installed. Otherwise hazardous chemicals may reverse flow through the hose into another operating area.

A number of hazards commonly can arise with valves:

Blocked-in pressure relief valves. Pressure relief valves play a critical safety role in almost all process facilities. Such valves simply must work. This means they never must be blocked in from the equipment item(s) that they’re protecting. Yet their very criticality demands that relief valves be routinely isolated or removed for maintenance and testing — both for the pressure at which they open and, more rarely, for their flow capacity.

Ideally, a relief valve wouldn’t be removed until the equipment being protected is shut down, purged, depressurized and air purged. Then, it can be taken out with impunity for bench testing. In practice, many organizations allow block valves to be placed underneath relief valves so that the valves can be removed while the plant remains in service. It’s critical these block valves are locked in the open position during normal operations.

The following guidance can help minimize risk associated with having block valves below relief valves:

• Install a bleed valve between the relief valve and block valve. If a rupture disc is used between the inlet block valve and the relief valve, place the bleed valve between the inlet block valve and the rupture disc.

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