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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19 Dec

Needle Valves- How Are They Different From Flow Control

needle-valves-how-are-they-different-from-flow-control

Needle valves are fittings which are designed with a slender and tapered point at the end of its valve stem. This is the valve’s stem which is actually lowered right through the seat, to block or restrict the flow.

Needle valves are used to ensure that the fluid passes through the valve, and actually turns 90 degrees before it passes through an orifice which serves as the seat. This seat comes with a rod having a cone shaped tip. The valves are actually used to regulate the actual flow of liquids as well as gases, at rates which are set at a low flow.

The fine threading design of the needle valve stem, along with its large seat area, ensures that precise resistance will follow. These valves are used to control and check the flow of liquid and gases into the delicate gauges of the system, which are at a high risk of suffering damage if subjected to extreme pressure.

These valves are also utilized in industrial equipment, where the flow needs to be brought to a final halt. They are also utilized where a precise adjustment of the flow is considered essential, or wherever a small and controlled flow is needed. These valves can easily be used as both on and off valves, along with the use in a system for throttling service.

Difference Between A Flow Control And Needle Valve

Both of these fittings are used to control and reduce the flow of air in the leg of a pneumatic circuit. This results in slowing down the actuator speed. While the basic function of both the fittings is flow restriction, the main difference is that the needle valve controls the flow of liquid or gas in both directions, while the flow control valve only ensures control in one direction only.

The needle valve is bi directional, while the flow control is one directional only. In both the fittings, there is a finely threaded stem, which allows the gradual adjustment of the quantity of controlled flow, which passes through the valve. This controlled flow enters the input port and travels through the orifice, which is actually sized by the tapered stem, and out through the output port.

The flow control fitting actually features a simple by pass feature, which is then allowed to ensure a rapid and free flow, out from the output port of the fitting, by means of the input port.

Two types of flow control fittings are used for the purpose of selection and placement, within a carefully designed electric circuit and which in turn greatly affects the overall functions of an actuator.

A meter out valve is a very common type of flow control fitting, which is used to control the actual exhaust flow being emitted by an actuator. While a meter in flow is a fitting which is used to restrict the overall flow of actuator. It is also termed as reverse flow control.

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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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27 Oct

How Does a Water Check Valve Work?

  how-does-a-water-check-valve-work

    Check Valves

  • Check valves are devices that are installed in pipelines in order to stop the backflow of material. In many cases, the material flowing through the pipe is water, and the check valve keeps the water from flowing backwards in the pipe. They do not require any outside activation, but are rather designed with mechanisms that block the water from reversing direction at a certain point.

     Uses

  • Water check valves are used in several different devices, and they are advantageous for several reasons. For example, they stop flooding or overflowing in sump pumps and water heaters. They protect equipment in control valves, strainers and flowmeters. They also save power, because the backwards-flowing water would use up electricity even when the device was off.

     Types

  • There are several types of check valves that are installed in water-using devices, but they are all based on the same underlying principle. A ball check valve consists of a ball held in place by a spring. When the water flows forward, the spring contracts and the ball is pushed away from the opening of the valve. Therefore, the water can flow through the valve. When the water stops flowing, the spring pushes the ball back into the opening in the valve and stops the water from flowing backwards.Swing valves work in a similar way. Instead of a ball, swing valves contain a small door on hinges. When the water flows by the door, the door swings back on its hinges and allows the water through. When the water stops flowing, the door slams shut and stops any backflow from forming.Similarly, a split-disk check valve contains a metal plate that folds in half as the water flows past it. When the water stops flowing, the plate springs open and blocks the entrance of the valve.

 

 

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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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