Valve Controls

Please see page 59 of this handbook:

http://www.documentation.frco.com/groups/public/documents/book/cvh99.pdf

linear : error increase then output increase linearly

equal : error increase then output increase in the form of percentage of error.

quick : on-off type,error increse then at one certain level output suddenly increse at full output

these articles is TRANSCRIBED from book

1- Introduction to Valve Coefficients

The measurement commonly applied to valves is the valve coefficient(Cv), which is also known as the flow coefficient. When selecting a valve for a particular application, the valve coefficient is used to determine the valve size that will best allow the valve to pass the required flow rate, while providing stable control of the process fluid. Valve manufacturers commonly publish Cv data for various valve styles, which are approximate in nature and can vary—usually up to 10 percent—according to the piping configuration or trim manufacture.

If the Cv is not calculated correctly for a valve, the valve usually experiences diminished performance in one of two ways: If the Cv is too small for the required process, the valve itself or the trim inside the valve will be undersized, and the process system can be starved for fluid. In addition, because the restriction in the valve can cause a buildup in upstream pressure, higher back pressures created before

the valve can lead to damage in upstream pumps or other upstream equipment. Undersized Cv's can also create a higher pressure drop across the valve, which can lead to cavitation or flashing.

If the Cv is calculated too high for the system requirements, a larger, oversized valve is usually selected. Obviously, the cost, size, and weight of a larger valve size are a major disadvantage. Besides that consideration, if the valve is in a throttling service, significant control problems can occur. Usually the closure element, such as a plug or a disk, is located just off the seat, which leads to the possibility of creating a high pressure drop and faster velocities—causing cavitation flashing, or erosion of the trim parts. In addition, if the closure element is closure to the seat and the operator is not strong enough to hold that position, it may be sucked into the seat. This problem is appropriately called the bathtub stopper effect.

2-Definition of Cv

One Cv is defined as one U.S. gallon (3.78 liters) of 60°F (16°C) water

that flows through an opening, such as a valve, during 1 min with a 1-

psi (0.1-bar) pressure drop. As specified by the Instrument Society of

America (ANSI/ISA Standard S75.01), the simplified equation for Cv is

Cv=flow*m √specific gravity at flowing temperature/pressure drop

3-Flow Characteristics

Each throttling valve has a flow characteristic, which describes the relationship

between the valve coefficient (Cv) and the valve stroke. In other words, as a valve opens, the flow characteristic—which is an inherence to the design of the selected valve—allows a certain amount of flow through the valve at a particular percentage of the stroke. This attribute allows the valve to control the flow in a predictable manner,which is important when using a throttling valve.

The flow rate through a throttling valve is not only affected by the flow characteristic of the valve, but also by the pressure drop across the valve. A valve's flow characteristic acting within a system that allows a varying pressure drop can be much different or can vary significantly from the same flow characteristic in an application with a constant pressure drop. When a valve is operating with a constant

pressure drop without taking into account the effects of piping, the flow characteristic is known as inherent flow characteristic. However, if both the valve and piping effects are taken into account, the flow characteristic changes from the ideal curve and is known as the installed flow characteristic. Usually, the entire system must be taken into account to determine the installed flow characteristic, which is discussed further in Sec. 2.2.5. Some rotary valves—such as butterfly and

ball valves—have an inherent characteristic that cannot be changed because the closure element cannot be modified easily. For that reason, rotary control valves in a throttling application can modify this inherent characteristic using a characterizable cam with the actuator's positioner, or by changing the shape of the closing device, such as a Vnotched ball valve. Quarter-turn plug and ball valves can modify the

characteristic by varying the opening on the plug (Fig. 2.1). On the other hand, linear valves usually have a flow characteristic designed into the trim, by determining either the size and shape of the holes in acage (Fig. 2.2) or the shape of the plug head (Fig. 2.3). The three most common types of flow characteristics are equal percentage, linear, and quick-open. The ideal curves for these three flow characteristics are shown in Fig. 2.4. However, the inherent characteristic of these curves can be affected by the body style and design, and

piping factors.

3-Equal-Percentage Flow Characteristic

Of the three common flow characteristics, the equal-percentage characteristic is the most frequently specified with throttling valves. With an equal-percentage characteristic, the change in flow per unit of valve stroke is directly proportional to the flow occurring just before the change is made. With an inherent equal-percentage characteristic, the flow rate is small at the beginning of the stroke and increases to a larger magnitude at the end of the stroke. This provides good, exact control of the closure element in the first half of the stroke, where control is harder to maintain because the closure element is more apt to be affected by process forces. On the other hand, an equal-percentage characteristic provides increased capacity in the second half of the stroke, allowing the valve to pass the required flow. An equal-percentage characteristic results in improved rangeability for a particular valve, as well as better repeatability and resolution in the first half of the stroke.The mathematical formula for an equal-percentage characteristic is

Q=

Q0 e^nL

dQ/dL=nQ

Q= flow rate , L = valve travel ,e= 2.718,Q0 = minimum controllable flow,n=constant

Although the flow characteristic of the valve itself is equal percentage,

the installed flow characteristic is closer to the linear flow characteristic.

This is usually the case when the process system's pressure

drop is larger than the pressure drop across the valve. Figure 2.5

shows two flow curves for an equal-percentage characteristic: the

inherent flow characteristic and the installed characteristic that takes

into account piping effects. The addition of the piping effects has a tendency

to move the flow characteristic away from the ideal equalpercentage

characteristic toward the inherent linear characteristic.

this for now

if you still interested i will send the linear and quick open tomorrow because i am to busy for now

i hope the previous messages help you to understand the idea

The simple answer is, different applications which require different input to output ratios.

A linear characteristic has a one to one relationship from imput signal to output flow, throughout the range of the valve. it is directly proportional.

Quick open is where you get a lot of action on the valve for a little bit of input.

chris