The primary function of a safety valve is to protect property and life.
Because a safety valve is often the last device to prevent catastrophic failure under pressure conditions, it is important that the valve works at all times i.e. it must be 100% reliable.
Safety valves should be installed wherever the maximum allowable working pressure of a system or pressure containing vessel is likely to be exceeded, in particular under fault conditions due to the failure of another piece of equipment in the system.
Pressure excess can be generated in a number of different ways including:
The term “Safety Valve” and “Relief Valve” are generic terms to describe a variety of pressure relief devices. A wide range is available based on the application and required performance criteria. The different designs are required to meet numerous national standards.
ASME / ANSI PTC 25.3 standards (USA)
Pressure relief valve – (This is a general term, which includes safety valves, relief valves and safety relief valves.)
A spring-loaded pressure relief valve which is designed to open to relieve excess pressure and to reclose and prevent the further flow of fluid after normal conditions have been restored. It is characterised by a rapid-opening 'pop' action or by opening in a manner generally proportional to the increase in pressure over the opening pressure. It may be used for either compressible or incompressible fluids, depending on design, adjustment, or application.
Safety valve - A pressure relief valve actuated by inlet static pressure and characterised by rapid opening or pop action.
Relief valve - A pressure relief device actuated by inlet static pressure having a gradual lift generally proportional to the increase in pressure over opening pressure.
Safety relief valve - A pressure relief valve characterised by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening pressure, depending on the application, and which may be used either for liquid or compressible fluid.
European standard EN ISO 4126-1
Safety valve - A valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a quantity of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent further flow of fluid after normal pressure conditions of service have been restored.
Basic Safety Valve Principles
This diagram represents a Safety valve in its very simplest form. The force acting on the inlet side of the disk is acting against the force applied by the spring plus the force applied by the back pressure on the top of the disk.
The valve remains closed when
(PI x Ab) < Fs + (PB x At)
Is in equilibrium when:
(PI x Ab) = Fs + (PB x At)
(PI x Ab) > Fs + (PB x At)
PI = Inlet pressure, PB = Back pressure
At = Top of disk area, Ab = Bottom of disk area
|Figure 1 - Simple Valve Model|
Things to notice from this design are that if PB is variable and quite large relative to PI, then this will cause the pressure at which the valve opens to vary which is undesirable. The following two designs (Fig 2 & Fig 3) are available that eliminate the effect of back pressure on the set pressure.
|Figure 2 - Fitted with belows||Figure 3 - Piston design|
The bellows prevents backpressure acting on the top side of the disk. In relation to the piston there is no top side within the main body of the valve hence again the back pressure cannot affect the set pressure.
Bellows failure is an important concern in critical applications where a very precise set pressure is required. In these cases some mechanism to detect a leak of process medium out of the top vent would be implemented.
Piston designs are not usually found in conventional Safety valves but are more common in Pilot Operated Safety valves.
Guidance on when to use Bellows
API 520 Practice Guidelines: a conventional design should not typically be used when the built-up backpressure is greater than 10% of the set pressure at 10% over pressure.
European standard EN ISO 4126: the built-up backpressure should be limited to 10% of the set pressure when the valve is discharging at the certified capacity.
Other Backpressure concerns
A large PB will also affect the flowrate of the valve when open.
The total backpressure is generated from two components, superimposed backpressure and the built-up backpressure.
In a conventional design (no bellows), the superimposed backpressure will affect the opening characteristic and set value, but the combined backpressure will alter the closing (blowdown) and re-seat value.
Overpressure is the percentage over the set pressure by which the valve is fully open. The blowdown is the percentage below the set pressure by which the valve is fully closed.
Figure 4 – Relationship between pressure and lift for a typical safety valve
Table 1 – Safety Valve Performance Summary
Table 2 – Safety Valve Standards
Components of a Safety Valve
Please note depending upon the manufacturer they may differ slightly to that shown below.
Figure 5– Typical Safety Valve Components
The basic elements of the design are:
Right angle pattern valve body
Inlet can be either a full nozzle or a semi-nozzle type. With a full nozzle design has the “wetted” inlet tract formed from one piece (as per figure 5) with the seat integrated into the top of the nozzle. The internal bore of the nozzle and the disc is the only part of the valve that is exposed to the process fluid with the valve in the closed position. A semi-nozzle design consists of a seating ring fitted into the body.
The disc is held onto the seat by the stem, with the downward force coming from the compression on the spring mounted in the bonnet. The amount of compression on the spring is adjusted by the spring adjuster under the cap.
|Figure 6 - Open Bonnet||Figure 7 - Closed Bonnet|
Unless bellows or diaphragm sealing is used, process fluid will enter the spring housing (or bonnet). The amount of fluid depends on the particular design of safety valve. If emission of this fluid into the atmosphere is acceptable, the spring housing may be vented to the atmosphere - an open bonnet. This is usually advantageous when the safety valve is used on high temperature fluids or for boiler applications as, otherwise, high temperatures can relax the spring, altering the set pressure of the valve. However, using an open bonnet exposes the valve spring and internals to environmental conditions, which can lead to damage and corrosion of the spring.
When the fluid must be completely contained by the safety valve (and the discharge system), it is necessary to use a closed bonnet, which is not vented to the atmosphere. This type of spring enclosure is almost universally used for small screwed valves and, it is becoming increasingly common on many valve ranges since, particularly on steam, discharge of the fluid could be hazardous to personnel.
Typical Cap Options
Open Lifting Lever
A lifting mechanism is recommended to test for correct valve operation at all times where corrosion, caking, or any deposit could prevent the opening operation.
Foreign particles can lodge under the seat of the valve when it discharges. The lifting lever allows you to lift the valve and flush the obstruction. Pressure relief valves for Section VIII require a lift lever on all air, steam, and hot water valves used at temperatures over 60 degC.
Typically used where periodic testing of the valve in location is desired to assure its operation.
With an Open lifting lever design, when the valve discharges, fluid media will escape into the atmosphere around the open lifting lever assembly.
If this is not desirable or when back pressure is present you would select a Packed Lifting Lever design.
|Figure 8 - With a Lifting Lever Fitted|
|Packed Lifting Lever|
As described above, this type is selected where leakage of the media to the atmosphere during valve discharge or during back pressure would be un-desirable.
A packed lever design is a completely sealed assembly.
|Some people consider a bolted and gasketed design better to the standard screw cap for applications with back pressure and / or vibration hence some manufacturers offer this as an option.|
|Gag Screw / Test Gag|
Under certain circumstances i.e. under the start-up conditions of a plant or to pressure test the system in a controlled environment, it may be required that the valve is prevented from opening.
This is achieved by screwing the bolt (shown on the wire) into the cap which screws down onto the stem and prevents it lifting.
Obviously it is important that test gags are removed prior to placing the valve into service.
Other Typical Options Available
The bellows is designed to cover the same area on the back of the disc equal to the seat area hence the back pressure will have no effect on the set pressure. See the previous section “Basic Safety Valve Principles”.
Bellows also protects the spindle, spindle guide and spring from the process medium.
|A micro switch is fitted on the exterior of the valve which is activated when the stem rises in the valve.|
A bolt on steam jacket for preserving the valve body temperature.
Typically used on fluids to prevent solidification of the flowing viscous fluids.
A disc is held against the nozzle by a spring, which is contained in a cast bonnet. The spring is adjusted by a compression screw to permit the calibration of opening or set pressure. An adjustable nozzle ring, threaded onto the nozzle, controls the geometry of the fluid exit control chamber (also known as a huddling chamber). The control chamber (huddling chamber) geometry is very important in controlling valve opening and closing pressures and stability of operation. The nozzle ring is locked into position by a ring pin assembly as shown in Figure 14 below.
|Figure 14||Figure 15 - Relationship of Nozzle Area to Control Chamber (Huddling Chamber)
Under normal system operation the valve remains in the closed position because the spring force (Fs) is greater than the system pressure acting on the internal nozzle seating area (PA). If system pressure increases to a point when these forces are equal, then the set pressure is reached. The disc lifts and fluid flows through the valve. When pressure in the system returns to a safe level, the valve closes.
Just prior to reaching set point, the pressure relief valve leaks system fluid into the huddling chamber. The fluid now acts on a larger area of the disc inside the huddling chamber (PAh), causing the valve to experience an instantaneous increase in the opening force. Refer to the figure 15 above to see relationship between Nozzle Area (A) and the Huddling Chamber Area (Ah). System pressure acting on the larger area will suddenly open the safety relief valve at a rapid rate.
Although the opening is rapid and dramatic, the valve does not open fully at set point. The system pressure must increase above set point to open the valve to its full lift and capacity position. Maximum lift and certified flow rates will be achieved within the allowable limits (overpressure) established by various codes and standards. All pressure relief ales are allowed an overpressure allowance to reach full rated flow. The allowable over pressure can vary from 10% to 21% on unfired vessels and systems, depending on the sizing basis, number of valves, and whether a fire condition is encountered.
Once the valve has controlled the pressure excursion, system pressure will start to reduce. Since the huddling chamber area is now controlling the exit fluid flow, system pressure must reduce below the set point before the spring force is able to close the valve. The difference between the set pressure and the closing pressure is called blowdown, and is usually expressed as a percentage of set pressure. The typical blowdown can vary from 7% to 10%, the industry standard.
The nozzle ring adjustment changes the shape and volume of the huddling chamber, and its position will affect both the opening and the closing characteristics of the valve. When the nozzle ring is adjusted to its top position, the huddling chamber is restricted to its maximum. The valve will usually pop very distinctly with a minimum simmer (leakage before opening), but the blowdown will increase. When the nozzle ring is lowered to its lowest position, minimal restriction to the huddling chamber occurs. At this position, simmer increases and the blowdown decreases. The final ring position is somewhere between these two extremes to provide optimal performance.
Liquid Service Operation
On liquid service, a different dynamic situation exists. Liquids do not expand when flowing across orifices, and a small amount of fluid flow across the nozzle will produces a large local pressure drop at the nozzle orifice. This local pressure drop causes the spring to reclose the valve if the fluid flow is minimal. Liquids leaking into the huddling chamber can quickly drain out by gravity and prevent fluid pressure from building up in the secondary area of the huddling chamber. Liquid relief valves are thus susceptible to a phenomenon called chatter, especially at low fluid flow rates. Chatter is the rapid opening and closing of the pressure relief valve and is always destructive.
Because of the difference in the characteristics of gases and liquids, some valve designs require a special liquid trim in order to meet ASME Code Section VIII performance criteria of full rated liquid flow at 10% overpressure.
With liquids since no visible or audible pop is heard at set point, the set pressure is defined as the pressure when the first heavy flow occurs (a pencil sized steady stream of water that remains unbroken for approximately one inch).