Watch Out With Piping, Valves and Hoses

Consider common dangers during process hazard analysis.

By Ian Sutton, AMEC Paragon

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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.

Piping
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.

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