Process safety management standards require plant managements to identify and address hazards. While every site poses unique risks, many hazards to do with piping, valves and hoses really don’t differ much from one facility to another. Therefore, to save time during process hazard analysis (PHA) and improve the quality of the evaluation, it’s useful to point out common dangers before PHA meetings start.
So, in this article, I’ll provide some guidance about the nature of these hazards.
Hazards associated with piping are particularly common with:
• hydraulic hammer;
• pig launcher and receivers;
• pressure in relief headers;
• overload of overhead vacuum lines; and
• underground piping.
Hydraulic hammer.Suddenly stopping or starting a liquid flow causes this phenomenon. If the valve in a line containing liquid is closed quickly, the entire volume of liquid in the line up to that valve is stopped quickly. The effect is to create a sudden pressure surge that can damage instruments and valves and, in extreme cases, cause the pipe to burst.
Hammer can occur in lines containing process vapors or low pressure steam. If the vapor or steam cools as it flows down the line, liquid forms in the lower section of the line and then flows with the gas or steam. Eventually the amount of liquid can become so great that it blocks the line in the same way that a valve would, thus creating the potential for hammer.
If hydraulic hammer is considered a potential problem and the valve is automatically actuated, the PHA team should think about recommending putting a restriction in the vent line from the actuator. This restriction would prevent the valve from closing too quickly.
Pig launchers and receivers. Use of pig launchers and receivers inherently poses hazards because process piping must be opened to the atmosphere and operators must be in the area (to insert and remove the pigs). Three risks require particular attention:
1. Operators and maintenance personnel face the potential of being exposed to large quantities of toxic or flammable materials if pigging equipment is prematurely opened.
2. If the operation isn’t properly conducted, the pig could accidentally shoot out of the launcher or receiver.
3. For larger lines, lifting a pig to put it into a launcher (or to remove it from a receiver) leads to the possibility of it being dropped onto someone or something.
You can ameliorate these hazards by using interlock systems and operating procedures that don’t allow the pig trap/receiver door to be open while the system is under pressure. Also, you can minimize risks of pigs impacting people or equipment by locating the trap/receiver away from normal operations and pointing doors toward a safe location.
Pressure in relief headers. Relief valves open at a set pressure. However, it’s important to realize that it’s differential pressure that actually causes the valve to open. Therefore, if the set point for a relief valve is, say, 50 psig, and pressure in the relief header is 10 psig, the relief valve will open when absolute pressure in the vessel it’s protecting is 60 psig. Such a scenario could occur if a plant-wide upset leads to multiple vessels discharging into the relief header simultaneously.
To get around this difficulty, some relief valves are designed to open at a specified absolute pressure. The PHA team should check which approach is being used and consider implications of multiple relief valves opening simultaneously (such as would happen if there were a large fire on the unit).
Overload of overhead vacuum lines. High capacity vacuum distillation columns typically have very large diameter overhead lines from the top of the column to the condenser because available pressure drop is so small. Because the system pressures are so low the piping wall can be quite thin. Therefore, if the line inadvertently is filled with liquid (either process fluids during a column upset or water during hydrotesting), the lines could collapse.
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.
• Use full-port gate valves for block valves.
• Don’t install block valves in the relief valve system of any boiler or steam generator constructed to the requirements of the ASME Boiler and Pressure Vessel Code, Power Boilers, Section I. Some state boiler laws also prohibit this practice.
• Ensure the block valves are locked open when the relief valve is in service.
• Frequently check the status of the block valves.
Vents and bleeders. A common hazard is the leak of a hazardous chemical from a vent or bleeder to the atmosphere. Causes of the leak can include: the vent or bleeder inadvertently being left open (or not completely closed); erosion or corrosion of internals of the vent or bleeder valve (leakage then can’t be stopped); and a sample point not properly shut.
Another threat is someone working on a vent valve pulling it off the pipe to which it was connected (this usually requires corrosion to have weakened the joint). Someone standing on a vent/bleeder line or inadvertently kicking it also can cause this type of rupture. So, to minimize such risks, it’s important to ensure that bleeder and vent valves discharge to a safe location and are properly maintained.
Critical control valves in manual. The PHA team should list all control valves that might be operated in manual mode. For each valve, the team should ask three questions:
1. Why is the valve in manual?
2. What would happen if it were switched to automatic?
3. Is the valve part of a safety shutdown system?
The last question is key. Operators may say that in an emergency they’d move the valve to its fail-safe position. However, humans are unreliable during an emergency but you can trust high integrity instruments and valves to properly operate.
Ian Sutton is safety engineering manager for AMEC Paragon, Houston. E-mail him at [email protected].