Control Valves
A valve is a control device that is utilized in adjusting the flow rate of a fluid in a pipe (Doddannavar, Barnard, and Ganesh, 2005). This implies that the flow rate will change as the valve opens and closes either partially or fully, thus providing some obstruction to some extent (Rennels and Hudson, 2012). The valve comprises a flow passage in which the flow area can be varied. Operation of valves in most scenarios is manual. In cases where the valves are operated automatically, electromechanical actuators are used. The electromechanical actuators can include electric motors or pneumatic actuators that are motorized by air pressure or hydraulic actuators motorized by liquid pressure such as water or oil (Rennels and Hudson, 2012). Valves normally provide different functions in a hydraulic circuit. These include providing direction, controlling pressure and controlling flow. These provide the most general classification of valves. Valves can also be categorized into various groups based on their design such as multi-turn valves, quarter turn valves, self-actuated valves and control valves.
Direction Control Valves
These valves provide or determine the path on which the liquid or gas will pass through in a hydraulic circuit system. These valves are used for on, off and changing direction purposes (Doddannavar, Barnard and Ganesh, 2005). Examples of these include gate valves and globe valves (Rennels and Hudson, 2012).
Pressure Control Valves
These valves have the purpose of shielding or providing resistance to the pressure that builds up in a hydraulic circuit. They are important since the variations in pressure surges in a hydraulic system generate instant pressure, which can be more than three times the normal pressure in the hydraulic system. Examples include check valves and relief valves (Rennels and Hudson, 2012).
Flow control valves
Flow control valves serve the purpose of regulating the control volume of fluids in pipe flow. These control valves can also be used to control and provide a constant, fluid flow rate in a hydraulic system (Doddannavar, Barnard, and Ganesh, 2005). These valves are responsible for regulating the flow or pressure of a fluid through the process of partially or fully opening or closing a section in the pipe flow. Rennels and Hudson (2012) provide some examples of these types of control valves. Globe, ball valves, butterfly valves, and angle valves are the most common valves that serve these purposes.
Multi-turn Valves
Multi-turn valves, also called linear motion valves, function through fully opening or closing the pipe diameter in a hydraulic system (Rennels and Hudson, 2012). The valves follow a linear motion that covers the entire pipe diameter where the motion is made possible using several turns of a screw mechanism. Examples include diaphragm valves, gate valves, globe valves, pinch valve, and needle valve (Rennels and Hudson, 2012).
Needle valve
The needle valve controls the volume of flow in small lines (Rennels and Hudson, 2012). These valves are used in flow metering applications where a constant calibrated flow is required to be maintained. Lewin (2001) notes that needle valves regulate the flow through either controlling high head flow in pipes or controlling terminal discharge. Rennels and Hudson (2012), note that, in addition to regulating the flow, the needle valves provide precise control for small flow rates. The design of the needle valves consists of a moderately small orifice that has a long conical seat. A needle-shaped plunger that is located at the end of a screw fits into this seat perfectly. Based on the Figure 1 below, turning the screw retracts the plunger causing the flow to occur between the seat and the plunger. The plunger has to be fully retracted for the flow to be possible. The accurate regulation of flow is possible due to the numerous turning of the fine-threaded screw as it retracts the plunger.
Figure 1: Needle Valve (adapted from Rennels and Hudson, 2012)
Cavitation in the needle valve is prevented by the presence of a sharp flow separation point that is visible at the seat of the body. Cavitation is also prevented by making the downstream cone angle of the needle be a little less compared to the downstream angle of the seat of the body (Lewin, 2001). The flow coefficient discharge for a needle valve at full valve opening (3600) is about 0.6, 0.4 at half valve opening (1800) and 0.26 quarter valve opening (900) (Lewin, 2001).
Diaphragm valve
This valve comprises of a valve body, a bonnet, and a diaphragm as shown in Figure 2. The diaphragm acting as a partition prevents the body fluid from entering the bonnet region and acts as a closing member (Rennels and Hudson, 2012). In some cases, a packing material may be required to act as a seal, but as Rennel and Hudson (2012) note, the dual functions of partitioning and sealing avoids the need for a packing material. A plunger is used to force the diaphragm against a wall in the valve body to close and stop the flow. This is achieved by lowering the plunger into the valve. The diaphragm valves have several advantages such as low cost, can be tightly closed, thus, used for accurate flow control and have low-pressure drops (Rennels and Hudson, 2012). The diaphragm valves also do not obstruct flow (Smith and Zappe, 2004).
Caution in maintenance needs to be taken since the flexible members in the diaphragm valve are susceptible to wear and tear. Replacements are made periodically (Rennels and Hudson, 2012). A disadvantage of the diaphragm valve is they are not suited to high-pressure applications and that high actuation forces may be needed to cut off flow. Operating temperatures are between -300 C and 1200 C and are limited by the elastomeric material used for the diaphragm (Dickenson, 1999). Skojkov (1997) supports this by noting that materials used in the manufacture of the diaphragm itself have relatively low temperature limits and have low tensile strength. Thus, any increase in temperature will cause a reduction in the strength of the diaphragm. The elastomeric material may include butyl rubber, neoprene, natural synthetic rubber, white butyl, viton and nitrile rubber (Dickenson, 1999). Being temperature dependent limits the sizes available for the diaphragm valves (Stojkov, 1997).
Figure 2: Diaphragm Valve (Adapted from Rennels and Hudson, 2012)
The flow coefficient values for the diaphragm valve are indicated below.
Globe Valve
The globe valve body is designed in such a way that the flow control element can move along the axis of the fluid path. This makes it be best suited for regulating flow and for starting and stopping flow (Stojkov, 1997). Smith and Zappe (2004), note that globe valves can only be used for starting and stopping flow as long as the valve can contain the flow resistance generated from the flow passage. Globe valves have a fixed solid barrier between the inlet and the outlet portion of the valve (Rennel and Hudson, 2012). The fluid flows through a hole in the barrier after which a closure member seals the hole. Skokjov (1997) refers to this closure member as a flow disc. Opening the valve causes opens up the disc allowing flow to move from the seat portion at once (Doddannavar, Barnard and Ganesh, 2005). This is what makes the globe valve suitable for throttling the flow. Smith and Zappe (2004) indicate that the movement of the disc in varies proportionally to the opening of the seat making it suitable for regulation of flow.
Figure 3: Globe Valve, Standard Pattern (Adapted from Smith and Zappe, 2004)
The standard globe valve pattern indicated above has a solid seat fixed at 900 to the inlet and outlets points (Rennels and Hudson, 2012). This causes a high-pressure drop suitable for regulation of flow but makes it unsuitable for the starting and stopping operations (Rennels, and Hudson, 2012).
The Y-pattern (Figure 6) globe valve has its stem angled (Dickenson, 1999). Rennels and Hudson (2012) indicate that the seat angle is at a 450 or a 600. This makes the flow path to be less twisted and has a reduced pressure drop compared to the standard globe valve. The Y-pattern of the globe valve reduces the pressure drop by streamlining the flow (Rennels and Hudson, 2012).
Figure 4: Y-pattern Globe Valve (Adapted from Dickenson, 1999)
Gate Valve
The gate valve (Figure 5) consists of a moving gate, which operates in the path of the fluid flow (Rennels and Hudson, 2012). Linear motion o the gate is normally greater or equal to the pipe diameter. As the name suggests, the main use of the gate valve is on and off operations. Rennels and Hudson (2012) indicate that because of the little restriction of fluid flow when the gate is open pressure drop that develops is low. Wearing of the seating faces is a common disadvantage of the gate valve (Smith and Zappe, 2004).
Figure 5: Gate Valve (Adapted from Rennel and Hudson, 2012)
Typical Flow coefficients for gate valves are as shown depending on the pipe diameter
Adapted from http://www.unimacvalves.co.in/valves_selection_guide.pdf
Quarter-Turn Valves
Butterfly valve
The butterfly valve (Figure 6) comprises of a large disk that rotates inside the pipe (Doddannavar, Barnard, and Ganesh, 2005). The diameter of the disc is less than or equal to that of the pipe (American Water Works Association, 2006). Rennels and Hudson (2012) describe the shape of the disc as being circular with its pivot axis being at a right angle to the direction of the flow in the pipe. Shutting off flow requires the disc to close against a ring seal. Smith and Zappe (2004) describe the butterfly valves as rotary valves in which the disc in the pipe is rotated through 900. Flow continues on either side of the disc, when split by the disc, in a full open disc position. In a closed position, the disc is perpendicular to the flow, therefore, stops the flow (American Water Works Association, 2006).
Types of the butterfly valves include the solid disc valve and the lattice blade valve that provides a stiffer disc assembly and lower loss coefficient of flow (Lewin, 2001). Resistance to flow when fully open is low. Smith and Zappe (2004) indicate that sensitive flow control in the butterfly valve is achieved when the valve is open between 150and700. Suitability to flow control of these valves is low because of the flutter of the blades and eddy shedding from the blade tips (Lewin, 2001). Flow control increases the wearing of the disc. Common applications of the butterfly valve include regulation of flow and as a stop valve (Stojkov, 1997).
Advantages of these valves include availability in large sizes, its quarter-turn operation makes it fast acting making it more useful as a stop valve and when the valve is lined, it can be used for transporting liquid slurries containing suspended particles (Stojkov, 1997). Lining is normally made of elastomeric material, which are corrosion and abrasion resistant to enable transportation of the liquid slurries (Smith and Zappe, 2004). However, the availability of butterfly valves is limited to the large sizes only. Application of the use of lined valves is dependent on the fluid temperature (Stokjov, 1997). Further, metal-seated valves do not achieve leak tight closing, thus not used as a stop valve.
Figure 6: Butterfly Valve (Adapted from Smith and Zappe, 2004)
Flow coefficients (K=0.855Cv) for a typical butterfly valve can be obtained from the figure below
Figure 7: Flow coefficients (K=0.855Cv) for a typical butterfly valve (Adapted from http://craneenergy.com/energy/products/quarter-turn-valves/resilient-seated-butterfly-valves/center-line-resilient-seated-butterfly-valves&page=4526E9AF-C260-92B6-8D93B8BF380244F2)
Ball Valve
The ball valve comprises of a spherical element that has a cylindrical hole to provide an allowance of flow when in the open position (Rennels and Hudson, 2012). Smith and Zappe (2004) describe the spherical element as being ball-shaped. The seat where the ball is supposed to be fitted is of the same shape, circumferentially, as the ball (Smith and Zappe, 2004). The ball is connected to shafts perpendicularly to the flow passage (American Water Works Association, 2006). The material used for the seating is normally non-corrosive metal (American Water Works Association, 2006). A main application of the ball valves is in situations where the velocity and pressure of the flow exceeds the capability of a butterfly valve.
Figure 8: Ball Valve, Adapted from Rennels and Hudson, 2012
Typical values of flow coefficients for ball valves are indicated below
Full Ball Valves
Reduced Ball valves
Adapted from http://www.engineeringtoolbox.com/ball-valves-flow-coefficients-d_223.html
Pipe Flow
Pipe flow is essential in engineering because of numerous applications it is used for such as water transportation and petroleum transportation (gas and oil) (Johnson, 1998). The flow rate in a pipe, therefore, depends on the velocity, pressure drop, and temperature. The types of flow that exist in pipes can be classified as either laminar flow or turbulent flow (Johnson, 1998). Laminar flow exists where the fluid flow is smooth, and any instability of flow may be due to the pipe wall. Low flow rates are responsible for laminar flow, where the pressure is also low. Increasing the pressure drop causes the flow rate to increase, resulting, or creating a scenario referred to as turbulent flow. Pressure mainly affects the fluid density, which is a critical factor in determining flow rate.
Oil and Gas Christmas Trees
The arrangement of control valves connected at the top of an oil or gas well to control the flow passage of the fluid is referred to a Christmas tree (Plunkett, 2009). Wright and Gallun (2008), also support this indicating that the Christmas tree may include valves, pipes, and fittings at the wellhead to control the flow. The arrangement of the valves in most cases will resemble a Christmas tree (Wright and Gallun, 2008). In subsea oil and gas production, the Christmas tree is installed at the head of each well (Crook, 2008). The Christmas tree utilizes several self-regulating valves that can shutoff flow from the oil reservoir (Crook, 2008). The idea behind using several autonomous valves is to ensure shut-off integrity is maintained incase a valve fails (Crook, 2008). Crook (2008) also adds that a sub-safety valve added on the Christmas tree functions as the last line of defense in case of an emergency. This may help prevent release of hydrocarbons into the sea.
Christmas trees in subsea pipeline are design carefully (Kang, Duan and Chen, 2011). Obtaining the best design requires optimization considering the water depth and temperature of in the sea. Where the Christmas trees have a likelihood of being damaged as be exposed on the surface, techniques such as plasma spraying of the sealing surfaces of the gate valves using hard corrosion resistant alloys is recommended (Mordynskii, Barukov, Isakahov and Komarcheva, 1987). The process of hardening the surface of the gate valves using the plasma powder has the effect of increasing the durability and life of the well-head equipment (Mordynskii, Barukov, Isakahov and Komarcheva, 1987).
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