Valve

VALVE: - Valves are component in fluid flow or pressure system that regulates either the flow or pressure of the fluid. This duty may involve stopping and starting flow, controlling flow rate, diverting flow preventing back flow, controlling pressure, or relieving pressure. These duties are performed by adjusting the position of the closure member in the valve. This may be done either manually or automatically.

FEED CHECK VALVE:-The function of the feed check valve is to allow the supply of water to the boiler at high pressure continuously and to prevent the back flow of water from the boiler when the pump pressure is less than the boiler pressure. It is fitted to the shell slightly below the normal water level of the boiler. The lift of the non-return valve is regulated by the end position of the spindle which is attached with the hand wheel. The spindle can be moved up or down with the help of hand wheel which is crewed to the spindle by a nut. But in case of boilers these valves movement are actuated by the help of an actuator. The valve is normally operated by the signal that comes from the differential pressure transmitter and in case the signal fails the valve remains in open position. The Figure below shows a simple feed check valve:

OPERATION OF CHECK VALVE:- Check valve operates in a manner that avoids:

The formation of an excessively high surge pressure as a result of the valve closing. Rapid fluctuation movements of the valve closure member.

GENERAL SELECTION CRITERIA:- There are certain basic criteria that has to be followed during valve selection.

Body pressure rating and limit. Size and flow capacity. Flow characteristics and rangeability. Temperature limits. Shutoff leakage. Pressure drop (shutoff and flowing) End connection requirements. Material compatibility and durability. Life cycle cost.

Each of them is defined briefly below:-

1) PRESSURE RATING :- Body pressure ratings of the valve are generally considered according to ANSI(American National Standard Institute) pressure classes. The most common ones for steel and stainless steel are ANSI classes 150, 300, and 600.

2) FLOW CAPACITY :- The criterion of capacity or size can be overriding constraint on selection. For very large lines sliding stem valves are much more expensive than rotary types. On the other hand for very small flows suitable rotary valve may not be available. So to reduce the confusion we can generalize the selection

process. For most general applications it makes sense both economically and technically to use sliding stem valves for the lower ranges, ball valve for intermediate capacities, and high performance butterfly valve for large sizes.

3) FLOW CHARACTERISTICS AND RANGEABILITY :- The next selection criteria, inherent flow characteristics refer to the pattern in which the flow at constant pressure drop changes according to valve position. Typical characteristics are quick opening, linear and equal percentage. The choice of characteristics has a strong influence on the stability or controllability of the process, since it represents the valve gain relative to travel.

Figure 1(% max flow v/s % of rated travel)

Parts and parcel of valve flow characteristics is range ability, which is the ratio of its maximum and minimum flow rates. Generally speaking, rotary valves, especially partial ball valves, have greater rangeabilty than sliding-stem varieties.

4) TEMPERATURE LIMITS :- Considerations here include the strength and ductility of the body material as well as the relative thermal expansion of the valve internal parts. Temperature limits may also be imposed due to disintegration of soft parts at high temperature or loss of resiliency at low temperatures. The soft material under consideration include various elastomers, plastics, and TFE. Typical upper limits for elastomers are in the 200 to 350oF range, and the general limit for PTFE is 450oF.

5) SHUTOFF LEAKAGE :- Some consideration usually must be given to a valve’s shutoff capability, which ordinarily is rated in terms of classes specified in ANSI. In actual service shutoff leakage depends on many factors including pressure drop, temperature, the condition of the sealing surface, and most importantly for sliding stem valves the force load on the seat. Tight shutoff is particularly important in high pressure valves since leakage can cause seat damage, leading to ultimate destruction of the trim. Special precautions in seat materials, seat preparation and seat load are necessary to ensure success. Valve users tend to overspecify shutoff requirements, incurring unnecessary costs. Actually very few throttling valves really need to perform double duty as tight block valve.

6) PRESSURE DROP :- The maximum pressure drop the valve can tolerate at shutoff and when partly or fully open is an important selection criterion. Sliding stem valves are generally superior in both regards because of rugged, well supported nature of their moving parts. Noise and cavitation are two considerations which, while unrelated are often grouped together because they both usually accompany high pressure drop and flow rates. Cavitation is the noisy and potentially damaging implosion of bubbles formed when the pressure of liquid momentarily dips below its vapour pressure through a constriction at high velocity.

Cavitation control and noise control trims for various degree of severity are widely available in regular sliding stem valves-at a progressive penalty in terms of cost and flow capacity.

7) END CONNECTION :- At some point in the selection process the valve’s end connection must be considered. In some situations, end situations can quickly limit the selection or dramatically affect the price.

8) MATERIAL COMPATIBILTY AND DURABILTY :- Material compatibility and durability are complex considerations. The issue may be corrosion by the process fluid, erosion by the abrasive material, flashin, cavitation, or simply a matter of process pressure and temperature. The material used for piping is a good predictor of control valve body material. However since the velocity is higher in valves, other factors must also be considered. When these items are included often valve and piping material will differ. Trim materials are usually a function of the body material, temperature range and qualities of the fluid. When a body material other than carbon, alloy or stainless steel is required, the use of alternate type of valves, such as line or bar stocked is considered. A number of factors must be considered to ensure that a material will perform properly in service. These fall primarily into two categories:- (a)the material suitability to function mechanically and (b) the material compatibility with the process environment.

Therefore based on the above explanation we can deduce:

The above chart shows that for a particular valve what is the pressure range, temperature range and pressure drop.

SPECIFICATION OF THE BOILER FOR WHICH THE VALVE SIZING IS DONE

Boiler plant concept:……………………………………………..……………1xMISSIONTM OLPressure gauge scale:............................................Multiscale (bar, MPa, kg/sq.cm)Thermometer calibration: .................................................................................. °CBoiler ambient temperature:.......................................................................... 45 °CBoiler design feed water temperature:........................................................... 80 °CBoiler operating feed water temperature: ................................................ 85-95 °CApprox. weight (excl. water):....................................................................24.5 tonApprox. weight (incl. water): ....................................................................35.5 tonBoiler design capacity: .............................................................................. 20.0 t/hBoiler operating capacity: ......................................................................... 18.0 t/hDesign pressure: .............................................................................................9 barWorking pressure: ...........................................................................................7 barTest pressure:........................................................................ 1.5 x Design pressureWorking temperature:................................................................................ 170.4 °CInsulation thickness: ..................................................................................... 75 mmAtomizing steam pressure: ..............................................................................6 barMax. atomizing steam consumption:............................................................75 kg/hAtomizing air pressure: ...................................................................................7 barMax. atomizing air consumption:...............................................................140 kg/hCapacity per pump: ................................................................................ 25.65 m3/hDelivery head: ................................................................................................13 bar

From the above data it can be inferred that the valve that is suitable for performing the job of a feed check valve is “SCREW DOWN NON RETURN GLOBE VALVE”. The following characteristics of the valve that supported the selection were: (a)applied in higher pressure and higher volume systems; (b)less suitable for viscous and contaminated fluid; (c)body material is cast iron, carbon steel, alloy steel, stainless steel; (d)excellent shutoff capability; (e)the only disadvantage it has high pressure drop.

Let us now see the designing steps of the “SCREW DOWN NON RETURN GLOBE VALVE”.

VALVE SIZING

Selecting the correct valve size for a given application requires a knowledge of the flow and process conditions . Sizing valve is based on a combination of theory and empirical data. The results are predictable, accurate and consistent. So to do valve sizing we can use the following formula:

Q = CV* SQRT {[P1-P2]/G}

Where,Q = flow rate = 25.65m3/hrCV = valve sizing coefficient, determined by testing(Flow coefficient of a control valve is expressed as the flow rate of water in m3/hr for a pressure drop of 1 bar across a flow passage)P1 = upstream pressureP2 = downstream pressureG = liquid specific gravity = 1 for water P = [P1-P2] = 0.59 (from the table above)

Calculating CV from the above equation we get CV = 33.39

Based on the above CV and pressure throughout the system the following valve is chosen.

NOMINAL PRESSURE: PN25

NOW VEIWING AT THE TABLE GIVEN BELOW WITH THE HELP OF PRESSURE WE CAN GET NOMINAL DIA OF THE PIPE, OD, PCD, NO OF HOLES AND DIA OF HOLES.

NOMINAL DIAMETER : DN65 ; OD : 185mm ; PCD : 145mm ; NO OF HOLES : 8 DIA OF HOLES : 18mm.

In the following table the standard selected is highlighted in red.

Diameters & Drilling of DIN Flanges ( mm )

Flange Nominal Diameter Pressure Rating (PN)

**Nominal BSP Pipe

Size PN6 PN10 PN16 PN25 PN40

DN 10 75 90 90 90 90 O.D.

3/8" 50 60 60 60 60 PCD (bolt circle)

4 x 11 4 x 14 4 x 14 4 x 14 4 x 14 # of holes x diam. of holes

DN 15 80 95 95 95 95 O.D.

1/2" 55 65 65 65 65 PCD (bolt circle)


DN 20 90 105 105 105 105 O.D.

3/4" 65 75 75 75 75 PCD (bolt circle)


DN 25 100 115 115 115 115 O.D.

1" 75 85 85 85 85 PCD (bolt circle)


DN 32 120 140 140 140 140 O.D.

1 1/4" 90 100 100 100 100 PCD (bolt circle)


DN 40 130 150 150 150 150 O.D.

1 1/2" 100 110 110 110 110 PCD (bolt circle)


DN 50 140 165 165 165 165 O.D.

2" 110 125 125 125 125 PCD (bolt circle)


DN 65 160 185 185 185 185 O.D.

2 1/2" 130 145 145 145 145 PCD (bolt circle)


DN 80 190 200 200 200 200 O.D.

3" 150 160 160 160 160 PCD (bolt circle)


DN 100 210 220 220 235 235 O.D.

4" 170 180 180 190 190 PCD (bolt circle)


DN 125 240 250 250 270 270 O.D.

5" 200 210 210 220 220 PCD (bolt circle)


DN 150 265 285 285 300 300 O.D.

6" 225 240 240 250 250 PCD (bolt circle)


DN 175 315 315 330 350 O.D.

7" 270 270 280 295 PCD (bolt circle)

8 x 22 8 x 22 12 x 26 12 x 30 # of holes x diam. of holes

DN 200 320 340 340 360 375 O.D.

8" 280 295 295 310 320 PCD (bolt circle)


DN 250 375 395 405 425 450 O.D.

10" 335 350 355 370 385 PCD (bolt circle)


DN 300 440 445 460 485 515 O.D.

12" 395 400 410 430 450 PCD (bolt circle)


DN 350 490 505 520 555 580 O.D.

14" 445 460 470 490 510 PCD (bolt circle)


O.D. = outside diameter PCD = bolt circle diameter PN = nominal pressure in bars Note: (1 bar = 14.5 psi = 0.1 MPa = 100 KPa = 1.02 Kgf/cm²)

ACTUATOR SIZING

Actuator are the distinguishing element between the valve and control valve. A control valve can perform its function only as well as the actuator can handle static and dynamic loads placed on it by the valve. Actuators are selected by matching the force required to stroke the valve with an actuator that can supply that force. For rotary valves a similar process matches the torque required to stroke the valve with an actuator that will supply that torque. The same fundamental process is used for pneumatic, electric, and electrohydraulic actuators. To select the appropriate actuator following factors to be taken into consideration:

1) Power source availability:- The power source availability at the location of the valve can often point directly to what type of actuator to choose. Actuators are either hydraulic, pneumatic or electric. Most of the actuators are pneumatic which work at pressure as low as 15psi to as high as 150psi.

2) Fail safe characteristics:- The reliability of the power source is quite high but even then the fail safe characteristics of a actuator is very important. Fail safe system store energy either mechanically in springs or pneumatically in volume tanks or hydraulic accumulators. When power fails the fail safe system are triggered to drive the valves to the required position and then maintain this position until resumption of normal operation.

The following are two type of spring and diaphragm actuators that are generally used for most of the control valves

AIR CLOSE, SPRING OPEN AIR OPEN, SPRING CLOSE

For FEEDCHECK VALVE we generally use pneumatic actuators and that too of the type “AIR CLOSE, SPRING OPEN” type. The reason behind using these kind of actuators is :

As the boiler requires continuous suppy of water, any failure if takes place in the actuator should not close the valve. If closure of the valve takes place then the boiler tubes will run dry and in presence of such a huge amount of heat they will melt away thereby causing fire in the boiler. So even if the actuator operating signal fails the valve should be continuously open to allow continuous flow of water thereby protecting the boiler.

3) Actuator capability:- An actuator must have sufficient thrust and torque for the application. For eg large valves requiring large torque will be limited to electric actuator. Conversely chosing an electric actuator for a small valve is not not a nice choice. Manufacturer is the best person to decide the type of actuator required for that kind of valve.

4) Control functions:- This function includes the actuator signal, signal range, ambient temperatures, vibration levels, operating speed, cycle frequency, and quality of control required. Low hysteresis and minimal dead band must be designed into actuators.

Globe ValvesThe force required to operate a globe valve includes:-- Force to overcome static unbalance of the valve plug-Force to provide a seat load- Force to overcome packing friction

- Additional forces required for certain specific applications or constructions

Total force required = A + B + C + D

A. Unbalance ForceThe unbalance force is that resulting from fluid pressure at shutoff and in the most general sense can be expressed as: Unbalance force = net pressure differential * net unbalance areaFrequent practice is to take the maximum upstream gauge pressure as the net pressure differential unless the process design always ensures a back pressure at the maximum inlet pressure. Net unbalance area is the port area on a single seated flow up design. Unbalance area may have to take into account the stem area depending on configuration. For balanced valves there is still a small unbalance area. This data can be obtained from the manufacturer.

B. Force to Provide Seat LoadSeat load, usually expressed in pounds per lineal inch of port circumference, is determined by shutoff requirements.Use the following guidelines to determine the seat load required to meet the factory acceptance tests for ANSI/FCI 70-2 and IEC534-4 leak classes II through VI. Because of differences in the severity of service conditions, do not construe these leak classifications and corresponding leakage rates as indicators of field performance. To prolong seat life and shutoff capabilities, use a higher than recommended seat load. If tight shutoff is not a prime consideration, use a lower leak class.

C. Packing FrictionPacking friction is determined by stem size, packing type, and the amount of compressive load placed on the packing by the process or the bolting. Packing friction is not 100% repeatable in its friction characteristics. Live loaded packing designs can have significantfriction forces especially if graphite packing is used.

D. Additional ForcesAdditional forces may be required to stroke the valve such as: bellow stiffness; unusual frictional forces resulting from seals; or special seating forces for soft metal seals as an example. The manufacturer should either supply this information or take it into account when sizing an actuator.

Actuator Force CalculationsPneumatic diaphragm actuators provide a net force with the additional air pressure after compressing the spring in air to close, or with the net precompression of the spring in air to open. This may be calculated in pounds per square inch of pressure differential.For example: Suppose 275 lbf. is required to close the valve calculated following the process described earlier.An air-to-open actuator with 100 square inches of diaphragm area and a bench set of 6 to 15 psig is one available option. The expected operating range is 3 to 15 psig. The precompression can be calculated as the difference between the lower end of thebench set (6 psig) and the beginning of the operating range (3 psig). This 3 psig is used to overcome the precompression so the net precompression force must be; 3 psig X 100 sq. in. = 300 lbf.This exceeds the force required and is an adequate selection. Piston actuators with springs are sized in the same manner. The thrustfrom piston actuators without springs can simply be calculated as: (Piston Area)(Minimum SupplyPressure) = Available Thrust(be careful to maintain compatibility of units)In some circumstances an actuator could supply too much force and cause the stem to buckle, to bend sufficiently to cause a leak, or to damage valve internals. This could occur because the actuator is too large or the maximum air supply exceeds the minimumair supply available. The manufacturer normally takes responsibility for actuator sizing and should have methods documented tocheck for maximum stem loads. Manufacturers also publish data on actuator thrusts, effective diaphragm areas, and spring data.

So finally, after looking into all the design aspects of sizing of valve and actuator screw down non return globe valve (straight) is chosen and direct acting pneumatic actuator is selected and the final assembly would look something like the figure below:

REFERENCES

www.google.com

www.wikipedia.org

Process/industrial instrumentation and control handbook.

Boiler room guide by Cleaver Brooks.

Emerson process management control valve handbook.

http://www.wikipedia.org/

http://www.google.com/

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Valve