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 P1 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, Pvapor, 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 DPcrit.
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:
where,
or terminal pressure drop;
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:
where,
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:
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:
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:
The equal percentage
characteristic is expressed mathematically as follows:
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:
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,
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