Wide Range                                                      

The Type 3500 Relief/Safety Relief valves are designed to have a short simmer, then to open rapidly to the full open position, and to re-seat at a controlled pressure. When the valve is in its fully lifted position, the discharge area is controlled by the bore of the nozzle, which ensures that flow calculations for various mediums can be reliably made.

Specification Valves are supplied in sizes one inch x two inch to eight inch x ten inch, orifices D through to T and can be manufactured in Cast Steel, Stainless Steel and any other materials to suit the application, with flanges to customers requirements. Valves can also be supplied with a packed lever or open lever lifting device, limit switch to indicate opening and closing of the valve, governing ring to limit adjustment of the spring to the set point, for ease of re-setting, balanced bellows when there is a variable back pressure.

What is a Relief Valve?                                                       

Generally speaking, safety relief valves have been around since the 1600s in more or less the same design concept. In its primary function, the pressure safety relief valve serves to protect life and property. Acting as a 'last resort', this fully mechanical valve is designed to open based on an over pressure situation within a process pressure system, thus not only protecting life but safeguarding the investment and plant itself.

Type 3500 is a high lift right angle safety relief / relief valve. It is designed to automatically discharge a fluid or gas at a desired set pressure

(A) An automatic pressure relieving device actuated by the static pressure upstream of the valve. Characterized by a rapid opening or pop action. Normally used on Gas or vapour service. SAFETY RELIEF VALVE.

(B) An automatic pressure relieving device actuated by the static pressure upstream of the valve. And lifts in proportion to the increase in pressure above the set pressure Normally used on liquid service. RELIEF VALVE The three main parts in a type 3500 are:-

      (1) Nozzle- which allows the fluid or gas to be piped away correctly.

      (2) Disc- which opens and closes the nozzles discharge.

      (3) Spring- which applies a force on the disc which opposes the force of the fluid below the disc.

Operation

When the force exerted by the pressure of the medium underneath the disc exceeds the force exerted by the spring, the disc begins to lift off the nozzle seat. The force exerted by the discharging medium on the surface of the chamber between the disc holder and the blow down ring produces a quick opening of the valve. This is due to the force and energy created by the discharging medium is much greater than the force exerted by the spring.

This should be with in 10% of the set pressure for Gas and 25% for liquid. The valve will stay open until the pressure drops and the force exerted by the spring is greater than the force generated by the medium acting on the disc holder. This should be with in 7% of the set pressure for Gas and 25% for liquid.

(A) The closer the blow down the ring the smaller the over pressure, the higher the blow down pressure.

(B) The further away the blowdown ring the higher the over pressure, the lower the  blowdown pressure.

(C) For liquids the blowdown ring is always set to the lowest position.

Size

The valves range in size from a 1” x 2” with a D’ orifice to 8” x 10” with a ‘T’ Orifice. With flanges ranging from 150 to 2500 ANSI or 6 to 40 DIN with flat face, raised face or RTJ ring face.

Manufacture

The valve can be manufactured in various materials such as Carbon Steel, Stainless Steel, Alloy Steel, Monel and Bronze. The valves can be fitted with bellows, which perform two functions

(1) It seals off the valve guide and spring from the fluid being discharged. This is important if the medium being discharged is corrosive.

(2) It avoids variation in the set pressure in the presence of a constant or variable back pressure. Because the bellows have an effective area equal to the area of the nozzle seating face.

The valves can be fitted with a manual lifting lever. The lifting lever comes in two types.

(1) An open lever. For non hazardous fluids.

(2) A closed packed lever. For hazardous fluids or back pressures.

The valve can be fitted with a micro switch. This is to notify the operator the valve is discharging.

Service

The valves can be used for a wide range of systems from air and steam to the Petrochemical industries.

Pressure Definitions

(1) Set Pressure
Predetermined pressure at which the valve disc starts to lift off its seat. Indicated by an audible discharge.

(2) Reseating Pressure
Pressure at which the valve closes after full discharge.

(3) Overpressure
The increase in pressure over the set pressure to which the valve should be fully open and discharging to the required capacity. The overpressure is expressed by percentage of the set pressures. Normally 10% of the set pressure for Gas and 25% for Liquid.

(4) Working Pressure
Pressure at which the system is working, this would be below the blowdown pressure of the safety relief valve.

(5) Design Pressure
Maximum pressure allowed for a system. The safety / relief valve would not be set above this pressure.

(6) Blowdown Pressure
The difference between the set pressure and the reseat pressure. Expressed by a percentage of the set pressure. Normally 7% of the set pressure for Gas and 25% for liquid.

(7) Back Pressure
Pressure acting on the outlet side of the valve. This can be caused by the medium when the valve is discharging. (BUILD UP BACK PRESSURE) or a back pressure resulting form design of the discharge system. This can be constant or variable back pressure. (Superimposed Back Pressure).

Safety Valve Terminology                                                       

Commonly-used terminology

 • Safety valve - steam applications, characterised by rapid, full opening or "pop" action.

 • Relief valve - liquid applications, the valves open in proportion to the increase of system pressure over the opening pressure.

 • Safety/relief valve - pressure relieving device suitable for use as a safety valve or a relief valve depending on its application.

 • Pilot-operated safety valve - in one such valve the spring provides +/-75% of the disk loading; the gas or vapour supplies the remainder through the pilot valve. When the pressure in the vessel reaches the set pressure, the pilot valve relieves the gas pressure (which contributes to the disk loading) to the atmosphere causing the safety valve to open wide. Both the pilot and the main valve contain flexible membranes and, consequently, are limited to the design factors of the membranes. These can be snap acting or modulating and are non-flowing.

 • Conventional safety valve - a conventional safety relief valve is a pressure relief valve characterised by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening, depending on the application. Such valves may be used either for liquid or compressible fluids.

 • Balanced safety valve - A balanced safety relief valve is a pressure relief valve, which incorporates a means of minimising the effect of backpressure on the operational characteristics (opening pressure, closing pressure, and relieving capacity).

 • Full nozzle - inlet flow passage; only the nozzle and disc insert are in contact with the process media when valve is in the closed position.

 • Semi nozzle - the nozzle, disc insert and part of the valve body are in contact with the process

   media.

 • Effective discharge area - the nominal orifice size listed in API-526, usually defined by a letter (D through T)

 • Actual discharge area - the measured minimum net area, which determines the flow through a valve.

 • Coefficient of discharge - ratio of the measured relieving capacity to the theoretical relieving

capacity.

 • Simmer - audible or visual escape of fluid between the seat and disc. Applies to valves on

compressible fluids, at around 98% of the set pressure.

 • Huddling chamber - annular pressure chamber located beyond the valve seat, this generates the pop characteristics.

Safety valve nomenclature

 • M.A.W.P. (Maximum Allowable Working Pressure) - maximum gauge pressure permissible at the top of a completed vessel for a designated temperature.

 • Operating pressure - working pressure in a pipe or vessel. ·Set pressure - pressure at which a relieving device opens and relieves.

 • Operating gap - difference between set pressure of the valve and the operating pressure of the vessel or system.

 • Overpressure - increase over set pressure of a relief device.

 • Accumulation - increase over M.A.W.P.

 • Blowdown - difference between set pressure and re-seating pressure of a safety valve.

 • Back pressure - pressure existing at the outlet of the pressure relieving device.

 • Back pressure is either - constant or variable.

 • Built-up pressure - the pressure existing at the outlet of a pressure relief device caused by the flow through that particular device into a discharge system.

 • Superimposed - the static pressure existing at the outlet of a pressure relief device at the time the device is required to operate. It is the result of pressure in the discharge system from other sources.

Full ASME & API Code Compliance                                                       

Because of their critical safety function, pressure relief valve design rules are very strict. Public safety laws in many countries require special inspection and verification of compliance with codes before allowing operation of the installed equipment.

The most widely used and recognised of these codes is the ASME Boiler and Pressure Vessel Code. One of the key features of the ASME Code is the rule for overpressure protection. These rules provide for the accreditation of manufacturers and the certification of pressure relief valves by tests in approved laboratories. Thus, the specification of ASME Code symbol stamped pressure relief valves assures the end-user that the performance requirements defined in the Code has been verified.

Application codes

The American Petroleum Institute has developed the most commonly applied standards and recommended practices for the petroleum and chemical industries in addition to the ASME Code. API Recommended Practice 521 provides excellent guidance for evaluating causes of overpressure and pressure relief systems.

API Recommended Practice 520 Part I is the design manual which is most widely used for the design. sizing and selection of components for pressure relief systems. Part II includes guidelines for recommended piping practices and methods for determining the reactive force created during valve discharge. If not properly evaluated, these reactive forces can cause chattering when conventional piping designs are applied. These high performance liquid service designs ensure smooth, stable operation and full relieving capacity on liquid service.

API also provides handling and storage recommendations. API Recommended Practice 526 provides an industry standard to manufacturers of flanged pressure relief valves and includes a common set of installation dimensions, pressure and temperature ratings, set pressure limits, capacities, and materials. This set of industry standards ensures that valves from different manufacturers will be interchangeable functionally and dimensionally. As many valves may not comply with this standard, these variables should be verified before substituting one model for another.

API Recommended Practice 527 provides a basis for testing and acceptance for set pressure and seats tightness of pressure relief valves.