23 Dec

Temperature Considerations for Pressure Relief Valve Application

temperature-considerations-for-pressure-relief-valve-application
The process of selecting or evaluating correct set pressure for pressure relief valve application is relatively straightforward – the set pressure of the pressure relief valve is compared to the pressure vessel maximum allowable working pressure (MAWP). The correct set pressure can be determined by using ASME Code rules and the general principle that at least one valve must be set at or below the MAWP.
The Code does not thoroughly explain temperature considerations, and application concerns related to temperature are just as important as proper selection of the correct set pressure. The purpose of this article is to examine some of these concerns when applying pressure relief valves in pressure vessel service.
One concern is selecting correct material for pressure relief valve construction. For pressure vessels, service temperature is considered part of the vessel design conditions, and the maximum temperature is used to select allowable stress limits for the chosen vessel material. Design temperature is recorded on the pressure vessel’s data report and nameplate (Section VIII, UG-119). Additionally, minimum design metal temperature (MDMT) is also considered in material selection and marked on the nameplate.
Pressure relief valve stamping rules do not require temperature marking on the nameplate; therefore, the valve manufacturer’s literature must be consulted to determine appropriate temperature limits for valve design. The valve must be applied using this data since it represents the mechanical limits of the design. When low temperatures may be encountered, materials appropriate for this application must be selected. Particular attention is called out for application of carbon and low alloy steels when used below -20° F. Impact testing of valve body materials may be required, and alternative materials with better impact resistance characteristics are often selected for low-temperature applications.
Once proper material for a valve is identified, temperature effects on valve operation and capacity must also be considered.
Valves are required to be tested with test fluid similar to the application fluid [UG-136(d)(4)]. Steam valves are tested with saturated steam by the manufacturer; if they are used in saturated steam service, performance inservice should be very similar to how the valve was set.
Valves for gas applications are set using ambient temperature air. Liquid service valves are set using water. The Code makes provisions for use of a cold set pressure which compensates for the difference between test medium temperature during the manufacturer’s original test and the valve’s actual temperature encountered inservice. Temperature of the system fluid, and possibly ambient operating temperature, should both be considered in application of the cold set pressure.
Cold set pressure is typically within several percentage points of the specified valve set pressure. For example, a Dresser 1900 series valve specified for 400° F service will have a multiplier of 1.013 applied to the required set pressure to achieve the desired set pressure inservice. (Reference: Dresser maintenance manual 1900-MM, dated 2009.) For most designs in elevated temperature applications, set pressure on the test stand will be higher than the set pressure inservice. This is because of thermal expansion of the valve’s bonnet (where the spring is located), and relaxation of the spring when it is heated above ambient temperature.
Both final set pressure (set pressure desired inservice), and cold set pressure are listed on the valve nameplate. Cold set pressure is listed as CDTP (Cold Differential Test Pressure). CDTP also includes a differential value to compensate for the effect of back pressure on a conventional type design (no bellows), with the back pressure compensation first considered.
Example using the aforementioned Dresser 1900 series valve:
A valve is required to open at 500 psig inservice where the service temperature is 400° F, and back pressure = 25 psig.
Differential set pressure = 500 psig – 25 psig = 475 psig
Temperature multiplier: 1.013
Cold differential test pressure = 475 psig x 1.013 = 481 psig
This valve would have a stamped set pressure of 500 psig and a CDTP of 481 psig. Back pressure of 25 psig would also be marked. Once the valve is inservice with specified back pressure applied at service temperature of 400° F, it should open at the desired set pressure of 500 psig.
When inservice inspections are performed, stamped set pressure value is compared to the vessel’s MAWP to determine whether set pressure was correctly specified. However, when this valve is tested on a test stand to verify inservice condition, measured set pressure should be compared to the CDTP to evaluate performance.
Interestingly, cold set pressure is usually not specified for valves inservice where temperatures are below ambient. The maintenance manual referenced above did not include a multiplier value for cold temperatures. Another manufacturer of valves for cryogenic services reported it did not use a cold set factor for valves in low temperatures. The reason: the pressure relief valve is normally installed in a location where the valve body is at ambient conditions, and the valves are not normally insulated. Therefore, the valve will operate at a temperature not much different than ambient.
Valves used in superheated steam service will also require a temperature correction, even though tested with saturated steam. Superheated steam is steam with energy added so the temperature is above saturation temperature for the given pressure. Under those conditions, a temperature correction is also applied to test set pressure, based upon the difference between saturated steam temperature and the superheat steam temperature. Manufacturer’s literature should always be consulted to determine the proper use of correction factors.
Valve capacity is also affected by temperature. Valve capacity markings are reported in standard units of pounds per hour of steam, standard cubic feet per minute of air at 60° F, or gallons per minute of water at 70° F. Service fluid temperature may often be different from standard conditions for capacity marked on the nameplate. A conversion from capacity on the valve nameplate to service fluid must be performed to determine whether valve capacity is correctly sized at service temperature conditions.
This calculation can be done using the guidance of ASME Code Section VIII, Appendix 11. Paragraph UG-125(a)(2) of ASME Code Section VIII indicates it is the pressure vessel user’s responsibility to select required pressure relief devices for a pressure vessel prior to initial service. Calculations used to select the pressure relief device should reflect sufficient capacity. When necessary, these calculations must be made available to the inspection organization.
Pressure relief valves are provided for the purpose of plant and personnel safety, and consideration of temperature effects on valve set pressure and capacity are some important aspects to be reviewed during selection and inservice inspection of pressure relief valves for pressure vessel applications.
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06 Dec

What Is A Plug Valve And When Is It Used?

what-is-a-plug-valve-and-when-is-it-used

We’ve previously discussed how to select specific valves for improving production time and MTBF and thought it would be a good time to review one of the most simple and economical valves available: the plug valve.

Plug Valve Basics

 A plug valve is shaped like a cylinder or cone and can be rotated inside the valve body to control flow of fluids. Plug valves have one or more hollow passageways often placed horizontally to allow ease of flow through the valve when open. The most common type of plug valve is the 2 port model with an open and closed position. The two ports are usually located on opposite sides of the valve with one passageway leading from inbound to outbound and the stem and handle located on the top. A plug valve also uses a quarter turn valve, which is useful where quick and frequent operation is essential. The valve ends can be flanged, hub type, or butt weld.

Plug Valve Types and Their Uses

 Although there are many types of plug valves, there are four general categories, each with their own benefits.

1. Lubricated Plug Valve

 As the name suggests, a lubricated valve uses a lubricant usually made up of a base oil and viscosity improver (like amorphous or fumed silica) that is injected under pressure between the plug face and body seat to reduce friction and seal ports. Valve manufacturers generally recommend lubricant suitable for the process fluid, and the valves often must be resealed after only a few cycles, and in some cases, after every cycle. Considered a high maintenance plug valve, they are often used in applications that have infrequent operations. Lubricated plug valves perform well in processes that utilize fluids which carry mildly abrasive particles such as dirty upstream applications, gas pipeline systems that need bypass valves, and as blow-down valves on valve stations and kicker valves.

2. Non-Lubricated Plug Valve

This plug valve utilizes a tapered or cone-shape that acts as a wedge and presses a polymeric sleeve against the body cavity. The use of the sleeve reduces friction between the plug mechanism and body.  Non-lubricated plug valves are often used instead of lubricated ones in applications where maintenance needs to be kept to a minimum, such special services in sulphur, hydrogen fluoride, or where liquids could be trapped or solidify and potentially jam the valve. However, non-lubricated plug valves are limited by temperature and chemical compatibility of the non-metallic materials they are made of.

The three main types of non-lubricated plug valves are:

Lift-type plug valve

Elastomer sleeved plug valve

Fully lined plug valve

3. Eccentric Plug Valve

The design of this plug valve uses a half plug that is advantageous for applications needing a higher seating force with minimal friction from open to closed position. The torque seated valves also feature improved shut off capabilities. Eccentric plug valves are used for a wide range of flow control and isolation applications including clean and dirty water, sewage, sludge and slurries, air, and other services.

4. Expanding Plug Valve

 This complex plug valve uses multiple components that allow the valve to mechanically expand and give it a true double block and bleed function in one valve. The plug valve uses a mechanism that rotates between the open and closed position and protects both seals from the flow path. During rotation there is no contact between body and seals, and slips expand onto the body seat when the valve is closed to avoid causing any wear or abrasion to the seals. They are often used to prevent product contamination in applications that do not require a double isolation.

In conclusion, plug valves have a number of useful applications, and new technology and designs are sure to give them an even wider variety of applications within many processes. And if you need more information on different types of valves, check out this piece on check valves or the rest of the site.

 

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03 Nov

What is a Plug Valve?

what-is-a-plug-valve

A plug valve is a simple type of valve that allows or blocks the flow of a liquid through a pipe. The plug itself is often shaped like an upside-down ice cream cone or a cylinder. A handle on top allows the user to turn the plug valve so it rotates and stops or starts the flow of liquid. A minimum of two holes, known as ports, must exist in the plug for liquid to flow. The ports are located on opposite sides of the plug, and when the plug is turned to the open position it creates a passage for the liquid to flow through.

This arrangement of two ports is known as a 2-port valve. The pipe in which the plug valve is installed has an area for liquid to flow in and another for liquid to flow out. When the 2-port plug valve is turned on, the ports line up with the holes in the pipe and the liquid flows straight through as if the plug valve was not there. Other, more complex plug valves known as 3-way valves allow liquid to flow to a combination of different ports. One port may connect to either of the opposite two ports, or all three may connect at once.

Two major categories of vales exist: stop valves and check valves. Stop valves follow a basic design structure to turn the flow of liquid on and off, or allow partial flow. Check valves, in contrast, allow liquid to travel only one way and often do not have a handle. Plug valves fall into the stop valve category.

Other stop valve types include gate valves and ball valves. Though similar to plug valves, the shape of the parts and how they work to stop the flow of liquid typically determines the specific group they belong to. Regardless of group, stop valves typically control liquid through the use of a handle.

Operation of a plug valve is simple, making it a common valve choice. The user twists the handle and the ports are moved away from the opening in the pipe. In certain valves, a full turn is not required. Quarter-turn valves, for example, only require the user to make a short, quarter turn to stop the flow of liquid. Often, plug valves can lessen the flow without turning off completely if the user turns the valve half-closed but leaves it open wide enough for some liquid to make it through the ports.

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29 Jan

DK-LOK Fittings and Valves for high-pressure equipment

dk-lok fittings valvesDK-LOK Fittings and Valves for high-pressure equipment.

  • Size Range: 1/16″ thru 1″
  • 316 SS, Carbon Steel, Brass, and Exotic Metals
  • Pressure Range: 1000 PSI thru 10,000 PSI
  • Full-Flow Quick Connects
  • Double Block and Bleed Valves
  • Swing-Out Ball Valves
  • Rising Plug Valves
  • Toggle Valves
  • Needle Valves
  • Relief Valves
  • Hose Adapters
  • Manifolds
  • Metering Valves
  • Purge Valves

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