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Farris: Frequently Asked Questions
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Can pressure relief valves be mounted horizontally?
Although valves can be physically mounted horizontally, they are designed to be mounted with the stems in the vertical position. Mounting the valve in other than the upright vertical position can lead to poor valve performance with excessive seat leakage.

3How much seat leakage can I expect from a pressure relief valve?
Pressure relief valves come in a variety of seat configurations. In normal operation, most valves with metal to metal seats are allowed a certain amount of leakage past the seats. American Petroleum Institute Standard 527, Seat Tightness of Pressure Relief Valves defines the most widely used method for testing seat tightness as well as the leakage rates. The rates are defined by pressure category and valve size and range from 20 to 100 bubbles per minute. The tests are conducted at 90 % of set pressure. Some valves like our 2600 and 2700 series can be supplied with optional "O" ring seats which allow for zero leakage at 90 % of set pressure. The 3800 Series pilot operated valve is supplied as standard with "O" ring seats and can safely handle operating pressures up to 90 % of set pressure with no leakage.

How often should a pressure relief valve be serviced?
The service and inspection interval for a pressure relief valve will vary depending on the installation, service conditions and type of valve in use. Each valve in a plant should be placed on a specific service schedule based on the users operating experience for each valve in a give application. Valves in corrosive applications will require a shorter maintenance interval than valves used on clean services. Other conditions that may effect the service interval are low differential between operating and set pressure, frequent valve operation, and excessive system vibration. American Petroleum Institute Standard 576, Inspection of Pressure-Relieving Devices can provide additional guidance.

What are the benefits of soft seat valves versus metal seat designs?
Pressure relief valves with soft seats offer the benefit of zero seat leakage when compared to valves with metal seats. A typical metal seat valve is allowed to leak a certain amount based on American Petroleum Institute Standard 527, Seat tightness of Pressure Relief Valves. Valves with "O" ring seats also offer the benefit of simpler maintenance by eliminating the necessity of lapping the seat and disc as replacement of the "O" ring and a light cleaning of the nozzle may be all that is necessary. Valves with "O" ring seats do have limitations based on the temperature range of the elastomer that is selected and the users' ability to find a compound that is compatible with the service fluid. Most "O" ring valves are limited to a maximum temperature of 450 F depending on the material selected. All Farris valves are available with an optional "O" ring seat in a variety of materials including Viton, Buna N, ethylene propylene, silicone and Kalrez.

When must I specify a lifting lever on a Pressure Relief Valve?
Section VIII of the ASME Code covering pressure vessels requires pressure relief valves to have lifting levers on air, steam, and hot water (over 140 F) service. There are however instances where this requirement may be ignored. ASME Code case #2203 allows for lifting levers to be ignored provided the following conditions are met.

  1. The user has a documented procedure and an associated implementation program for the periodic removal of the relief valve for inspection, testing, and repair.
  2. The user specifies that no test lever be supplied.
  3. The user shall obtain permission to omit the lifting lever (device) from the authority having jurisdiction over the installation of pressure vessel.

These rules apply to valves installed in ASME Section VIII applications. All valves used in ASME Section I (Boiler) applications require lifting levers except for those valves used in organic fluid vaporizer service.

When must I specify the use of a Balanced Bellows pressure relief valve?
Balanced bellows as used in our 2600 Series BalanSeal design are generally specified for a variety of reasons. The most prominent is to nullify the effects of back pressure in the discharge system on the valves set pressure. They are also used to protect the principal guiding surfaces, spring, and valve top works from coming in contact with a corrosive fluid. This may allow for the use of less expensive metallurgy as the bellows isolates the trim from the fluid. A balanced bellows valve should always be used when the variation in back pressure exceeds 10 % of set pressure.

When should I specify a pilot operated relief valve?
Unlike spring loaded valves that use a spring to hold the seat closed, pilot operated valves use the system pressure to provide closing force. The Farris 3800 series pilot operated valve consists of a main valve that is controlled by the all stainless steel PCF3 pilot control. Pilot operated valves offer the advantage of actually becoming tighter as the operating pressure approaches the set pressure. In addition, they are typically offered with soft seats that allow for zero seat leakage up to 90 % of set pressures. Pilot operated valves such as the 3800 series are normally used where maximum seat tightness is required and where the operating pressure is close to set pressure. Generally they are limited to operating temperatures of 450 F as they use elastomeric seats and seals. In addition, care should be used on dirty services as pilot operated valves are more prone to fouling than spring loaded types.

For more information call 440- 838-7690 or email farris@curtisswright.com


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