By Gobind Khiani – Consulting Fellow-Piping/Pipelines
Safety relief valves should be connected as direct and close as possible to the vessel being protected. The valve should be mounted vertically in an upright position either directly on a nozzle from the pressure vessel or on a short connection fitting that provides a direct, unobstructed flow between the vessel and the valve. Installing a safety relief valve other than in this recommended position will adversely affect its operation.
When designing the inlet piping, one of the considerations is to ensure that the pressure drop in piping is minimized. EN ISO 4126 recommends that the pressure drop be kept below 3% of the set pressure when discharging.
Note: The valve should never be installed on a fitting having a smaller inside diameter than the inlet connection of the valve.
Where safety valves are connected using short ‘stub’ inlet piping, it must be of the same size as the safety valve inlet. For larger lines or any line incorporating bends or elbows, the branch connection should be at least two pipe sizes larger than the safety valve inlet connection, at which point it is reduced in size to the safety valve inlet. Excessive pressure loss can lead to ‘chatter’, which may result in reduced capacity and damage to the seating faces and other parts of the valve. To reduce the pressure loss in the inlet, the following methods can be adopted:
Increase the Diameter of the Pipe
Ensure that any corners are suitably rounded, EN ISO 4126: Part 1 recommends that corners should have a radius of not less than one quarter of the bore.
Reduce the inlet pipe length i.e. install the valve at least 8 to 10 pipe diameters downstream from any converging or diverging ‘Y’ fitting, or any bend/elbow/tee reducer etc.
Never install the safety valve branch directly opposite a branch on the lower side of the steam line.
Avoid take-off branches in the inlet piping, as this will increase the pressure drop.
Outlet piping, also known as discharge piping systems, are open or closed. An open system goes directly into the atmosphere whereas a closed system goes into a manifold along with and/or without a safety valve.
Discharge piping should be simple and direct. A ‘broken’ connection near the valve outlet is preferred wherever possible. All discharge piping should be run as direct as is practicable to the point of final release for disposal. The valve must discharge to a safe disposal area. Discharge piping must be drained properly to prevent the accumulation of liquids on the downstream side of the safety valve.
The weight of the discharge piping should be carried by a separate support and be properly braced to withstand reactive thrust forces when the valve relieves. The valve should also be supported to withstand any swaying or system vibrations.
If the valve is discharging into a pressurized system be sure the valve is a ‘balanced’ design. Pressure on the discharge of an ‘unbalanced’ design will adversely affect the valve performance and set pressure.
Fittings or pipe that have a smaller inside diameter than the valve outlet connections must not be used.
The bonnets of balanced bellows safety valves must always be vented to ensure proper functioning of the valve and to provide a telltale in the event of a bellows failure. Do not plug these open vents. When the fluid is flammable, toxic or corrosive, the bonnet vent should be piped to a safe location.
It is recommended that discharge piping should rise for steam and gas systems, whereas for liquids, it should fall. Horizontal pipework should have a downward gradient of at least 1 in 100 away from the valve ensuring that any discharge will be self-draining. It is important to drain any rising discharge pipework. Vertical rises will require separate drainage.
Note: all points of system drainage are subject to the same precautions, notably that valve performance must not be affected, and any fluid must be discharged to a safe location.
It is essential to ensure that fluid cannot collect on the downstream side of a safety valve, as this will impair its performance and cause corrosion of the spring and internal parts. Many safety valves are provided with a body drain connection, if this is not used or not provided, then a small-bore drain should be fitted in close proximity to the valve outlet.
One concern in closed systems is the pressure drop or built-up backpressure in the discharge system. That drastically affect the performance of a safety relief valve. The EN ISO 4126: Part 1 states that the pressure drop should be maintained below 10% of the set pressure. To achieve this, the discharge pipe can be sized.
Manifolds must be sized so that in the worst case the piping is large enough to cope without backpressure. The volume of the manifold should ideally be increased as each valve outlet enters it, and these connections should enter the manifold at an angle of no greater than 45° to the direction of flow.
The manifold must also be properly secured and drained where necessary.
For steam applications, it is generally not recommended to use manifolds, but they can be utilized if proper consideration is given to all aspects of the design and installation.
The reaction forces are typically small for safety valves with a nominal diameter of less than 75 mm, but safety valves larger than this usually have mounting flanges for a reaction bar on the body to allow the valve to be secured.
These reaction forces are typically negligible in closed systems, and they can therefore be ignored.
Regardless of the magnitude of the reaction forces, the safety valve itself should never be relied upon to support the discharge pipework itself and a support should be provided to resist the weight of the discharge pipework. This support should be located as close as possible to the centreline of the event pipe.
Changeover valves permit two valves to be mounted side by side, with one in service and one isolated. This means regular maintenance can be carried out without interruption of service or the vessel being protected. Changeover valves are designed in such a way that when they are operated, the pass area is never restricted.
Changeover valves can also be used to connect safety valve outlets so that the discharge pipework does not have to be duplicated. The action of both inlet and outlet changeover valves must be limited and synchronized for safety reasons. This is usually by means of a chain drive system linking both handwheels.
Consideration must be made to pressure loss caused by the changeover valve when establishing the safety valve inlet pressure drop, which should be limited to 3% of the set pressure.
Seat tightness is an important consideration when selecting and installing a safety relief valve, as not only can it lead to a continuous loss of system fluid, but leakage can also cause deterioration of the sealing faces, which can lead to premature lifting of the valve.
The seat tightness is affected by three main factors:
1. The characteristics of the safety relief valve
2. Installation of the safety relief valve
3. Operation of the safety relief valve
Characteristics of the Safety Relief Valve
For a metal-seated valve to provide an acceptable shut-off, the sealing surfaces need to have a high degree of flatness with a very good surface finish. The disc must articulate on the stem and the stem guide must not cause any undue frictional effects. Typical figures required for an acceptable shut-off for a metal seated valve are 0.5 μm for surface finish and two optical light bands for flatness. In addition, for a reasonable service life, the mating and sealing surfaces must have a high wear resistance.
Unlike ordinary isolation valves, the net closing force acting on the disc is relatively small, due to there being only a small difference between the system pressure acting on the disc and the spring force opposing it.
Resilient or elastomer seals incorporated into the valve discs are often used to improve shut-off, where system conditions permit. It should be noted, however, that a soft seal is often more susceptible to damage than a metal seat.
Safety Relief Valve Installation
Seat damage can often occur when a valve is first lifted as part of the general plant commissioning procedure, because very often, dirt and debris are present in the system. To ensure that foreign matter does not pass through the valve, the system should be flushed out before the safety valve is installed and the valve must be mounted where dirt, scale and debris cannot collect.
It is also important on steam applications to reduce the propensity for leakage by installing the valve so that condensate cannot collect on the upstream side of the disc. This can be achieved by installing the safety valve above the steam pipe
Emissions from Safety Relief Valves
Noise emissions are one of the concerns, although discharge from a safety valve should not occur frequently. Should it occur, the noise generated can often be significant. It is therefore necessary to determine the sound level of safety valves to ensure that relevant health and safety regulation levels are not exceeded.
When a sonic flow nozzle discharge occurs. The approximate value of the sound level in decibels at a flange outlet can be calculated.
As an example, the safety relief valves installed on the Sump Pump discharge for blocked flow that operate at the set pressure on downstream up to 1900kPag. It is not recommended to go with the long pipe run on the discharge of that safety relief valve that goes all the way back to the meter drain system. A better design would be to dump the safety relief valve line straight back into the sump tank, saving pipe, EHT and insulation costs and efficient operation.
REFERENCES & PICTURE COURTESY
1. Crosby – Pressure Relief Valve Engineering Handbook Anderson Greenwood Crosby – Technical Seminar Manual | Spirax Sacro – Alternative Plant Protection Devices and Terminology Campbell Sevey Inc.
2. API code committee members on Relief Valves (API520, 521, 526 & 527).
3. ISO code committee members on Relief Valves (EN ISO 4126).
4. Taylor Valves Inc.
ABOUT THE AUTHOR
Gobind N. Khiani, a UCalgary alumnus with a BSc in Civil Engineering and MSc in Mechanical Engineering has a proven track record in technical and value engineering and holds a Fellowship in Engineering and an MBA. Currently, he holds the position of Vice Chairman of the Standards Council of Canada. He has done peer review on Emissions Management regulatory documents for Environment and Climate Change Canada and participated in research and development initiatives for Emissions Management and Reduction Programs, Alberta and Canada’s Oil Sands Innovation Alliance (COSIA) – a world-leading innovation alliance that set the model for sharing intellectual-property to accelerate environmental performance.