Study the Characteristics of Control Valve

Objective:

To study the characteristic of three control valves viz. quick opening, linear and equal percentage.

Theory:

Control valves are valves used to control conditions such as flow, pressure, temperature, and liquid level by fully or partially opening or closing in response to signals received from controllers that compare a "set point" to a "process variable" whose value is provided by sensors that monitor changes in such conditions. Control Valve is also termed as the Final Control Element. The opening or closing of control valves is usually done automatically by electrical, hydraulic or pneumatic actuators. Positioners are used to control the opening or closing of the actuator based on electric, or pneumatic signals. These control signals, traditionally based on 3-15psi (0.2-1.0bar), more common now are 4-20mA signals for industry, 0-10V for HVAC systems, and the introduction of "Smart" systems, HART, Fieldbus Foundation, and Profi bus being the more common protocols. Some of the control valves available are Reverse Double-Ported Globe-Style Valve Body, Three-Way Valve with Balanced Valve Plug, Flanged Angle-Style Control Valve Body, Valve Body with Cage-Style Trim, Balanced Valve Plug, and Soft Seat.

The set up:

The set up consists of three pneumatic control valves (CV), a compressor that generates pneumatic pressure of 0-100 PSI, a water tank, a rota meter, some hand valves (HV), a water pump and some pipe fittings. The flow elements here is the water, stores in a small reservoir or tank. And the valves used are Quick Opening, Linear and Equal percentage. All are connected to the same pressure source. Every valve controls the flow of water separately (one valve at a time). The pump is used to pressurise the water to flow in the pipe through valves. The rota meter is used as water flow rate indicator. Each control valve has a stem position pointer that slides over an mm scale. The hand valves are connected with the pipes to control the water flow and air pressure. The schematic is given below:

Experiment-1

Objective:

To study the quick opening control valve characteristics and to calculate the gain at various conditions.

Apparatus Required:

Compressor

Procedure:

1. Hand valve HV1, HV2 and HV5 are opened and HV3, HV4 and HV7 are closed.
2. Air regulator is adjusted to give 15 PSI output.
3. The unit is switched ON and pump speed is adjusted for maximum flow.
4. The hand valve HV1 is adjusted to give maximum flow through control valve.
5. Flow through the rotameter and stem position are noted.
6. ·         Air regulator output is varied, and the corresponding flow rate is measured.
7. Step 5 and 6 are repeated for the different pressure and the readings are tabulated.
8. Gain of the control valve at various operating pressure is calculated using the formula: 

\[Gain = \frac{{Q/{Q_{\max }}}}{{S/{S_{\max }}}}\]

Where,

Q= Instantaneous flow in LPH

Q_max = Maximum flow in LPH

S = Instantaneous stem position in mm

S_max = Maximum stem position in mm

Table 1:

Sl. no	Control air pressure	Stem position	Flow rate	S/Smax	Q/Qmax	Gain

Experiment 2:

Objective:

To study the Equal Percentage control valve characteristics and to calculate the gain at various conditions.

Apparatus Required:

Compressor

Procedure:

1. Hand valve HV1, HV3 and HV6 are opened and HV2, HV4, HV5 and HV7 are closed.
2. Air regulator is adjusted to give 15 PSI output.
3. The unit is switched ON and pump speed is adjusted for maximum flow.
4. The hand valve HV1 is adjusted to give maximum flow through control valve.
5. Flow through the rotameter and stem position are noted.
6. ·         Air regulator output is varied, and the corresponding flow rate is measured.
7. Step 5 and 6 are repeated for the different pressure and the readings are tabulated.
8. Gain of the control valve at various operating pressure is calculated using the formula: 

\[Gain = \frac{{Q/{Q_{\max }}}}{{S/{S_{\max }}}}\]

Where,

Q= Instantaneous flow in LPH

Q_max = Maximum flow in LPH

S = Instantaneous stem position in mm

S_max = Maximum stem position in mm

Table 2:

Sl. no	Control air pressure	Stem position	Flow rate	S/Smax	Q/Qmax	Gain

Experiment 3:

Objective:

To study the Linear control valve characteristics and to calculate the gain at various conditions.

Apparatus Required:

Compressor

Procedure:

1. Hand valve HV1, HV4 and HV7 are opened and HV2, HV3, HV5 and HV6 are closed.
2. Air regulator is adjusted to give 15 PSI output.
3. The unit is switched ON and pump speed is adjusted for maximum flow.
4. The hand valve HV1 is adjusted to give maximum flow through control valve.
5. Flow through the rotameter and stem position are noted.
6. ·         Air regulator output is varied, and the corresponding flow rate is measured.
7. Step 5 and 6 are repeated for the different pressure and the readings are tabulated.
8. Gain of the control valve at various operating pressure is calculated using the formula: 

\[Gain = \frac{{Q/{Q_{\max }}}}{{S/{S_{\max }}}}\]

Where,

Q= Instantaneous flow in LPH

Q_max = Maximum flow in LPH

S = Instantaneous stem position in mm

S_max = Maximum stem position in mm

  

Table 3:

Sl. no	Control air pressure	Stem position	Flow rate	S/Smax	Q/Qmax	Gain