Before we start, let’s take a look at what are considered “repairs” to safety and safety relief valves. Repairing a safety valve or safety relief valve could involve a number of operations, such as replacement, re-machining or cleaning of any critical part, lapping of the seat and disk — or any other operation which may affect the flow passage, capacity, function or pressure retaining integrity of the valve. Even disassembling, reassembling or making adjustments which affect a valve’s function should be considered a repair. This does not include testing or adjustments of new valves upon initial installation.
There are many types and designs of safety valves. Even though individual components may vary by design, the basic functions of the parts do not change from one valve to the next. However, this does not mean all valves are alike and repaired alike. Each valve design has its own maintenance requirements and specifications, and they should be carefully studied before a repair authorized.
In general, safety valves have the following components:
Nozzle — A pressure-containing element which constitutes the inlet flow passage.Seat — A pressure-containing component which makes contact between the fixed and moving portions of a valve. Guide — A guiding surface for the disk to ensure proper alignment of the seating surfaces. Stem — A means to connect the disk to the lifting device. Spring — Used to provide the static force necessary to keep the disk in contact with the seat until the set pressure is reached. Compression Screw — The external adjustment used to establish the valve’s set pressure.
Improper repairs to safety valves can be divided into two basic categories:
An example of poor workmanship could be the overtightening of a valve in a vice during its disassembly. This could result in cracking the body or deforming a part, which could lead to misalignment or binding once the valve is reassembled.
One of the most commonly overlooked issues of this nature are cracked components. It is imperative that repair organizations incorporate thorough, non-destructive examination into their repair procedures to ensure flaws do not go undetected.
Possible Effects of Improper Repairs
The most detrimental effect of a poor repair job is a reduction in a valve’s relief capacity. After all, the primary function of a safety valve to relieve at a sufficient capacity to prevent exceeding a certain value above the maximum allowable working pressure of the vessel. Other effects might be a shift in the valve’s set pressure or blowdown outside the specified tolerances of the code.
Operationally, the valve’s moveable components may hang up, flutter or chatter, and leakage or excessive simmering may be present. The valve’s lift might be restricted in some way. All of these items, either individually or collectively, could have an overall effect on relieving capacity.
The Bottom Line
Each repair organization is required to ensure their personnel are knowledgeable and fully qualified. It is essential that each organization establish an effective quality control system to ensure that valves repaired have been returned to performance and conditions equivalent to the standard for new valves.
By combining the use of competent repair personnel with effective quality control systems — and conducting repairs in accordance with the National Board VR program, it’s possible to overcome the problems caused by improper repair of safety valves.
Information from this article was taken from the National Board of Boiler and Pressure Vessel Inspectors. More information can be found at www.nationalboard.org.