Control Valve

10.Control valves

 

10.1. Valve in control loop

 

The final control element is the last element of the closed control loop that implements the control action. It receives the output signal (control or actuating signal) from a controller and adjust accordingly the value of the manipulated variable by changing the amount of matter or energy entering the process. The following definition of a control valve (from Process Instruments and Controls Handbook, Ed. by Considine D.M., McGraw-Hill Book Company, 1985, p. 19.4) is used by the Scientific Apparatus Makers Association (SAMA):

 

“A control valve is a valve with a pneumatic, hydraulic, electric or other externally powered actuator that automatically, fully or partially opens or closes the valve to a position dictated by signals transmitted from controlling instruments.”

 

Control valves are used primarily to throttle energy in a fluid systems and not only for shut-off purposes. Their internals must withstand high fluid velocity and turbulence for long periods without maintenance.

 

 

10.2. Pneumatic operating valve

 

Figures 10.1 and 10.2 shows a schematic design of a pneumatic valve. This valve is an air-operated device which controls the flow through an orifice by positioning appropriately a plug. In other words, it is a variable orifice in a line.

 

The plug 1 is placed in the orifice 2 of the valve and attached to the end of the stem 3. The orifice is placed inside the body of the valve 4 made of cast iron, alloy steels, alloy steels plus corrosion-resistant alloys, or bronze. The upper part of the final control element is an actuator 5. A diaphragm 6 divides this actuator in two chambers. The upper end of the stem is supported on the diaphragm. When the air pressure (the output signal from a pneumatic controller) above the diaphragm increases, the diaphragm deflects and the stem moves downwards thus restricting by the plug flow of the fluid through the orifice. This type of a pneumatic valve is called “air-to-close” valve. When the air pressure goes down the stem under the action of a spring 7 will move upwards, thus opening the orifice. There is another type of valves, which operate in opposite action, ie, when the air pressure increases the plug opens the orifice. Such valves are called “air-to-open” valves. If the air pressure varies from 20 to 100 kPa the plug is moved from a fully open to fully closed position.

 

 

10.3. Actuator

 

This is a mechanism that physically opens or closes the valve. The actuator is an intermediate device between the convereted control signal and the final control element.

 

Figure 10.1. Air-to-close pneumatic valve.

 

Figure 10.2. Air-to-open pneumatic valve.

An actuator must provide an accurate output position proportional to the input signal in spite of various forces:

 

· inertia forces;

· static friction forces;

· thrust forces caused by weight and unbalanced fluid pressure.

 

Position of the stem depends on:

 

            · the effective area of the diaphragm;

            · the pressure on the diaphragm;

            · the force of the spring at the particular degree of compression;

            · the pressure drop across the seats;

            · the hysteresis due to guides and packing;

            · the weight of moving parts.

 

 

10.4. Valve positioner

 

Valve positioner (Figure 10.3) is a control device designed to impart sensitivity to the valve and to enshure accurate positioning as dictated by a control signal.

 

Figure 10.3. Valve positioner.

Below are several conditions when valve positioners should be used:

 

            · high pressure across valve;

            · control with wide throttling range;

            · high-pressure applications with tight packing;

            · valves handling sludge or solids in suspension;

 

There are two types of positioners:

 

· deflection (or position) balance positioner;

· force balance positioner.

 

 

Figure 10.4. Block-diagram of the control valve assembly.

 

 

10.5. Basic valve types

 

Below is the list of names of basic valve types:

 

· ball valves;

· butterfly valves;

· diaphragm valves;

· gate valves;

· plug valves.

 

 

10.6. Flow of fluid through valves

 

Due to the valve body restriction the inlet pressure of the stream P₁ will drop to the value of P₁’ (so-called inlet pressure loss), then due to the major valve restriction the pressure will drop further to the value of P_{1 min}. The place of this minimum pressure is called vena contracta. Then, this pressure will be recovered to the value of P₂’ and due to the body restriction this pressure will fall to P₂ (so-called outlet pressure loss). When conditions of the flow through the valve are non-critical we have either liquid (for the case of a liquid stream) or gas (in the case of a gas stream) all distance through the valve. If flow conditions are critical then we get two cases:

 

· the pressure of the liquid when passing through the valve can fall down to the vapor (saturating) pressure of this liquid, P_vapor, at the current temperature. In this case liquid (say, water) starts to boil with the formation of bubbles, which collapse and condense while flowing downstream to the zone of higher pressures (P₂’ and P₂). This collapse of bubbles develops localised pressures of up to 690 MPa. This undesired hydrodynamic phenomenon which is called cavitation causes rapid wear of the valve trim, valve body and outlet piping, develops severe noise and vibration.

 

· the pressure of the gas when passing through the valve can fall down to its critical pressure. This will cause gas sonic velocity. As a result vibration and severe noise are developed.

 

 

Figure 10.5. Variation of the fluid pressure when passing through a control valve.

In order to eliminate the possibility of cavitation one may install a control valve at the lower place in the piping system, thus increasing the inlet pressure P₁ and, therefore, reducing the pressure drop across the valve to the value less than DP_crit.

 

The constant of the valve for conditions of flow can be determined experimentally for various conditions. The differential pressure across the valve can be calculated from the analysis of the decreasing of pressure along the pipes (hydraulic gradient method). Thus evaluated valve flow coefficient is used for selection of the desired type of the valve from the tables for standard valves.

 

 

10.6.1. Choked flow

 

As the liquid passes the point of greatest restriction inside the control valve, its velocity reaches a maximum and its pressure falls to a minimum. If the pressure falls below the liquid’s vapor pressure, vapor bubbles form within the valve.  Increasing the pressure drop across the valve beyond this point where vapor bubbles form has no effect on the flow. The pressure drop at which choked flow begins is called the terminal pressure drop.

 

Choked flow produces either flashing or cavitation

 

 

10.6.2. Flashing

 

If the pressure downstream of the valve is below the liquid’s vapor pressure, the vapor bubbles persist in the liquid. This is flashing.

 

Requirements for occurrence of flashing:

 

· the fluid at the inlet must be in all-liquid condition, but some vapor must be present at the valve outlet;

· the fluid at the inlet may be in either a saturated or a subcooled condition;

· the valve outlet pressure must be either at or below the vapor pressure of the liquid.

 

Flashing effects:

 

· material damage is associated with the formation of sand-blasted surfaces;

· decreased efficiency, in other words flashing (as well as cavitation) reduces the ability of the valve to convert pressure drop across the valve into mass flowrate.

 

 

10.6.3. Cavitation

 

Cavitation is a two stage process:

 

· the formation of voids and cavities within the liquid system;

· the collapse or implosion of this cavities back into in all-liquid state.

There are two type of cavitation, namely, gaseous and vaporous. Both type of cavitation require the presence of some nucleating agent for their inception. This nuclei (contains either vapor or dissolved gas) will enlarge into finite cavities within the liquid. In carefully degassed liquid cavitation may be significantly delayed.

 

Requirements for occurrence of cavitation:

 

· the fluid at both the inlet and outlet must be in all-liquid condition;

· the liquid must be subcooled state at the inlet, because if the liquid will be in a saturated state, then any pressure drop across the valve will cause the presense of vapor downstream;

· the valve outlet pressure must be either at or above the vapor pressure of the liquid.

 

Evidences of cavitation:

 

· noise. At fully developed cavitation it sounds like a gravel psees through the valve;

· vibration. Depend on the mass of the system, how well the system components are anchored, whether valve-mounted instruments are vibration-sensitive;

· material damage. Damage of valve plugs, development of eroded holes through the valve body, damage of the guiding surfaces and valve plug seating surfaces, etc.

 

Among theories of cavitation we can mention two which are the most eccepted:

 

· high pressure shock waves from the bubbles exploding in close vicinity of the solid part of the valve strike that surfaces and destroy them.

 

· chemical theory suggests that when a solid surface undergoes the strike from shock waves, temperature of these surfaces increases due to absorption of energy, and as the result a chemical reaction between the flowing fluid and the surface material occurs.

 

 

10.6.4. Maximum practical pressure drop across liquid valves

 

In order to avoid cavitation and flashing damage a maximum allowable pressure drop across the valve must be identified:

 

                                                ,                                  (10.1)

where,

 

              - the maximum allowable differential pressure for sizing purposes,

or terminal pressure drop;

                             - the valve recovery coefficient from the manufacturer’s catalogue;

                     - absolute inlet fluid pressure;

               - absolute fluid vapor pressure at the inlet temperature;

                 - the critical pressure ratio;

                - pressure drop between valve inlet and vena contracta at

choked flow.

 

 

10.7. Valve characteristics

 

The characteristic of a control valve is the relationship between the valve position and the flowrate through the valve in the following form:

 

            ,                                   (10.2)

where,

 

                       - fluid flowrate through the control valve;

                  - maximum fluid flowrate through the control valve;

                          - stem displacement from the closed position;

                    - maximum stem displacement;

                                - fractional stem displacement.

 

 

 

 

3.7.1. Inherent valve characteristics

 

The inherent characteristic of a valve is obtained when there is a constant pressure drop across the valve for all valve positions, the process fluid is not flashing, cavitating or approaching sonic velocity (choked flow), and the actuator is linear (valve stem travel is proportional to the controller output).

 

10.7.1. Quick opening valve characteristics

 

Control valves with this characteristic provide a large change in flowrate for a small change in valve position. This characteristic is used for on/off or two-position control systems in which the valve must move quickly from open to closed or vice versa. (see line a in figure below).

 

Such a valve may allow 90% of maximum flowrate with only a 10% travel of the stem.

 

Figure 10.6. Inherent Valve Characteristics.

 

10.7.2. Linear valve characteristics

 

This characteristic (see line b in figure above) provides a linear relationship between the valve position and the flowrate and is described by the following mathematical relationship:

                                                .                                                         (10.3)

 

This is the ideal situation when the valve alone determines the pressure drop.

 

 

10.7.3. Square root valve characteristics

 

This valve characteristic is described by the following mathematical relationship:

 

                                                .                                           (10.4)

 

Line c in the above figure represents this type of valve characteristic.

 

10.7.4. Equal percentage valve characteristics

 

This valve characteristic provides equal percentage changes in flowrate for equal changes in valve position. An equal percentage valve is designed to operate between a minimum flowrate, , and a maximum flowrate, . The rangeability of the valve can be determined as follows:

 

                                                            .                                                     (10.5)

 

The equal percentage characteristic is expressed mathematically as follows:

 

                                                            .                                              (10.6)

 

This type of control valve does not shut off the flow completely in its limit of stem travel.  represents the minimum flowrate when the stem is at one limit of its travel. Line d in the above figure shows the equal percentage characteristic for the case when , line e for the case when .

 

A fractional change in valve stem position produces a proportional change in the valve flowrate.

 

Typical values are:                   .

 

 

10.7.5. Hyperbolic valve characteristics

 

Hyperbolic valve characteristic has the following mathematical representation:

 

                                                .                                             (10.12)

 

and is shown as the line f in the above figure.

 

10.8. Control valve selection

 

10.8.1. Metric valve flow coefficient,

 

A valve may be regarded as a variable orifice. It is often convenient to express the relationship between pressure drop and flowrate through a valve by a flow coefficient. Values of this coefficient are determined by testing and may not valid for all flow conditions.

 

Metric valve flow coefficient, , represents the flow in cubic meters per hour which can be passed by the valve when a pressure drop across the valve is equal 1 bar.

 

So,                                           ,                                                        (10.13)

where,

 

                              - metric valve flow coefficient;

                     - mass flowrate of fluid;

                     - pressure drop across the valve;

                                    - the fluid density.

 

Below are suggestions which are need to be used when choosing control valves:

 

· use data and corresponding sizing equations from a recognised manufacturers catalogue;

· always check that the valve catalogues you intend to use are current and upto date;

· always seek advice from the manufacturer’s representative about a particular valve selection