10 basic knowledge of flow control valve selection

Composition:

The flow regulating valve is composed of a valve body, an executive actuator, and its accessories.

Working principle:

The pneumatic diaphragm actuator of the flow control valve has two forms of positive action and reverse action; as the signal pressure increases, the pushrod moves downward as a positive effect, otherwise, the pushrod moves upward as a counter effect; usually, the standard signal pressure is 20~100KPa; with The maximum pressure of the positioner is 250KPa. There are six types of stroke (mm): 10; 16; 25; 40; 60; 100.

Compared with pneumatic actuators, what are the characteristics of electric actuators, and what output types are there?

The driving source is electric power, which is simple and convenient, with high thrust, torque, and rigidity. But the structure is complicated and the reliability is poor. It is more expensive than pneumatic in small and medium specifications. It is often used in places where there is no gas source or where strict explosion-proof and flame-proof are not required. There are three output forms: angular travel, linear travel, and multi-turn.

What are the advantages of the straight-through single-seat control valve? What occasions is it suitable for?

• The leakage flow is small because there is only one spool, it is easy to ensure the sealing. The standard discharge flow rate is 0.01%KV, and further design can be used as a shut-off valve.
•  The allowable pressure difference is small, and the thrust is large due to unbalanced force. The △P of DN100 valve is only 120KPa.
•  The circulation capacity is small. The KV of DN100 is only 120. It should be applied to occasions where the leakage is small and the pressure difference is not large.

What are the characteristics of the straight-through double-seat control valve? Where is it used?

• The allowable pressure difference is large because it can offset many unbalanced forces. DN100 valve △P is 280KPa.
• Large circulation capacity. The KV of DN100 is 160.
• The leakage is large because the two-valve cores cannot be sealed at the same time. The standard discharge flow rate is 0.1%KV, which is 10 times that of a single-seat valve.
  It is mainly used on occasions with high-pressure differences and low leakage requirements.

What are the main advantages of sleeve control valves?

• Good stability. Because the valve plug throttling replaces the valve core valve seat throttling, and the valve plug is provided with a balance hole, the unbalanced force of the medium acting on the valve plug can be reduced. At the same time, the guide surface between the sleeve and the valve plug is large, and the unbalanced force changes little, so it is not easy to cause the valve core to vibrate.
• High interchangeability and versatility. As long as the sleeve is replaced, different flow coefficients and different flow characteristics can be obtained.
• The allowable pressure difference is large and the thermal expansion has little effect. The balance principle of the sleeve valve with balance hole is the same as that of the double seat valve, so the allowable pressure difference is large. Because the sleeve and valve plug are made of the same material, the expansion caused by temperature changes is basically the same.
• The throttle window provided by the sleeve has two types: large opening and small hole (jet type). The latter has the effect of reducing noise and vibration and is further improved to become a special low-noise valve. It is suitable for occasions where the pressure difference between the front and rear of the valve is large and the noise is low.

In addition to single-seat valves, double-seat valves, and sleeve valves, which other valves have regulating functions?

Diaphragm valves, butterfly valves, O-shaped ball valves (mainly cut-off), V-shaped ball valves (large adjustment ratio and shearing effect), and eccentric rotary valves all have adjustment functions.

What do the adjustable ratio R, theoretical adjustable ratio, and actual adjustable ratio of the control valve mean?

The ratio of the maximum flow rate and the minimum flow rate that the regulating valve can control is called the adjustable ratio R.
When the pressure difference between the two ends of the valve remains constant, the ratio of the maximum flow rate to the minimum flow rate is called the ideal adjustable ratio. In actual use, the pressure difference between the two ends of the valve changes, and the adjustable ratio at this time is called the actual adjustable ratio.

What is the flow coefficient C, Cv, KV value of the regulating valve?

The flow capacity of the regulating valve is expressed by the flow coefficient.

• Definition of engineering unit system Cv: When the regulating valve is fully opened, the pressure difference between the front and rear of the valve is 1kgf/cm2, and the number of cubic meters per hour that water with a temperature of 5-40°C passes.
• Definition of British unit system C: When the regulating valve is fully opened, the pressure difference between the front and rear of the valve is 1bf/in2 (1 degree 60. F of water passes through the number of US gallons per minute.
• International System of Units KV: When the regulating valve is fully opened, the pressure difference between the front and rear of the valve is 100kPa, and the number of cubic meters per hour that water with a temperature of 5-40℃ passes.
  Cv=1.17 KV
  KV＝1.01 C

What are the seven steps to determine the size of the regulating valve?

1. Determine the calculated flow-Qmax, Qmin
2. Determine the calculated pressure difference-select the resistance ratio S value according to the characteristics of the system, and then determine the calculated pressure difference (when the valve is fully opened);
3. Calculate the flow coefficient-select the appropriate calculation formula chart or software to find the max and min of KV;
4. KV value selection-according to the KV max value in the selected product series, the KV closest to the first gear is used to obtain the primary selection caliber;
5. Checking the opening degree-when Qmax is required ≯90% valve opening; when Qmin is ≮10% valve opening;
6. Checking calculation of actual adjustable ratio-general requirement should be ≮10; R actual> R requirement
7. Determination of the caliber-if it is unqualified, reselect the KV value and verify again.

What are the influences of the various parts of the control valve on the output force of the actuator?

• Overcome the static imbalance force experienced by the spool.
• Provide the tight pressure of the valve seat load.
• Overcome the friction of the packing.
• Additional forces are required for specific applications or structures (such as bellows, soft seals, etc.).

What does the flow opening and flow closing of the regulating valve mean?

Definition: At the throttle, the flow direction of the medium is the same as the valve opening direction, which is called flow open; otherwise, it is called flow closed.

It is for the direction of the medium flow and has nothing to do with the mode of action of the regulating valve, air opening, and air closing. The importance of flow direction is that it affects stability, leakage and noise.

Which valves need to be selected for flow direction? how to choose?

Single-seal control valves such as single-seat valves, high-pressure valves, and single-seal sleeve valves without balance holes need to be flowed.
There are pros and cons of flowing open and closed. The flow-open type valve works relatively stable, but the self-cleaning performance and sealing performance are poor, and the life is short; the flow-close type valve has a long life, the self-cleaning performance and the sealing performance are good, but the stability is poor when the stem diameter is smaller than the valve core diameter.
Single-seat valves, small flow valves, and single-seal sleeve valves are usually selected to flow open, and flow closed when the flushing is severe or self-cleaning is required. The two-position type quick opening characteristic control valve selects the flow closed type.

Three main factors for selecting the actuator of the control valve:

• The output of the actuator should be greater than the load of the regulating valve and should be reasonably matched.
• When checking the standard combination, whether the allowable pressure difference specified by the regulating valve meets the process requirements. When the pressure difference is large, the unbalanced force on the spool must be calculated.
• Whether the response speed of the actuator meets the requirements of the process operation, especially the electric actuator.

What are the auxiliary devices (accessories) of the pneumatic control valve? What role does each play?

• Valve positioner-used to improve the working characteristics of the regulating valve and achieve correct positioning;
• Valve position (stroke) switch-display the upper and lower stroke working position of the regulating valve;
• Pneumatic retaining valve-keep the valve’s current position when the air source fails;
• Solenoid valve-to realize automatic switching of the gas path. Two-position three-position for single air control; two-position five-way for double air control;
• Manual mechanism-manual operation can be switched when the system fails;
• Pneumatic relay-to speed up the action of the pneumatic film actuator and reduce the transmission time;
• Air filter pressure reducer-for air source purification and pressure regulation;
• Air storage tank-When the air source fails, the valve can continue to work for a period of time, usually with three-stage protection.

Under what circumstances need to use a valve positioner?

• Occasions, where the friction is large and precise positioning, are required. For example, high temperature and low-temperature control valves or control valves with flexible graphite packing;
• The slow process needs to increase the response speed of the control valve. For example, the adjustment system of temperature, liquid level, analysis, and other parameters.
• Where it is necessary to increase the output force and cutting force of the actuator. For example, single-seat valve with DN≥25, double seat valve with DN>100. When the pressure drop at both ends of the valve △P>1MPa or the inlet pressure P1>10MPa.
• When the split-range regulating system and regulating valve are in operation, sometimes it is necessary to change the air opening and air closing modes.
• Where it is necessary to change the flow characteristics of the regulating valve.

What are the characteristics of self-operated control valves? What do K and B in the model of self-operated pressure regulating valves stand for?

Self-operated control valve is also called direct-acting control valve. It does not need any external energy, and integrates the three functions of measurement, adjustment, and execution into one, and uses the energy of the adjusted object to drive its action. It has the characteristics of simple structure, low price, reliable action, and so on.
It is suitable for occasions where flow rate changes are small, regulation accuracy is not high, or instrument air supply is difficult.
Self-operated control valves can be divided into pressure, liquid level, temperature, and flow control valves according to their uses. At present, the most produced are pressure regulating valves and nitrogen sealing valves.
K——Press-open type, used for pressure relief, stable in front of the valve;
B——Pressure-closed type, used for voltage stabilization, stable after valve.

Basic knowledge of valve

1. The basic parameters of the valve are: nominal pressure PN, nominal diameter DN
2. The basic function of the valve: cut off the connected medium, adjust the flow rate, and change the flow direction
3. The main ways of valve connection are: flange, thread, welding, butt clamp
4. The pressure of the valve-the temperature grade indicates: under different materials and different working temperatures, the maximum allowable working pressure without impact is different
5. There are two main systems for pipe flange standards: European system and American system. The flange connection dimensions of the two systems are completely different and cannot be mutually matched;

The pressure levels of the European standard and the American standard are:

European standard nominal pressure: PN0.25, 0.6, 1.0, 1.6, 2.5, 4.0, 6.3, 10.0, 16.0, 25.0, 32.0, 40.0MPa;

American standard nominal pressure: PN1.0 MPa (CIass75), PN2.0 MPa (CIass150), PN5.0 MPa (CIass300), PN11.0 MPa (CIass600), PN15.0 MPa (CIass900), PN26.0 MPa ( CIass1500), PN42.0 MPa (CIass2500).

The main types of pipe flanges are integral (IF), plate flat welding (PL), flat welding with neck (SO), butt welding with neck (WN), socket welding (SW), screw (Th), butt welding ring Loose sleeve (PJ/SE)/(LF/SE), flat welding ring loose sleeve (PJ/RJ) and flange cover (BL), etc.

Flange sealing surface types mainly include full surface (FF), raised surface (RF), concave surface (FM), convex surface (M), tenon surface (T), groove surface (G), ring connection surface (RJ), etc.

Commonly used (general) valve code:

1. The valve type codes Z, J, L, Q, D, G, X, H, A, Y, S represent gate valve, globe valve, throttle valve, ball valve, butterfly valve, diaphragm valve, plug valve, stop Return valve, safety valve, pressure reducing valve, trap
2. The connection codes of the valve 1, 2, 4, 6, and 7 respectively indicate: 1—internal thread, 2—external thread, 4—flange, 6—welding, 7—pair clamp
3. The code of transmission mode of the valves 9, 6, 3 respectively indicate: 9—electric, 6—pneumatic, 3—turbine worm
4. The valve body material codes Z, K, Q, T, C, P, R, V respectively indicate: gray cast iron, malleable cast iron, ductile iron, copper and alloys, carbon steel, chromium-nickel stainless steel, chromium-nickel molybdenum stainless steel, Chromium-Molybdenum Vanadium Steel
5. Valve seat seal or lining code R, T, X, S, N, F, H, Y, J, M, W respectively represent: austenitic stainless steel, copper alloy, rubber, plastic, nylon plastic, fluoroplastic, Cr series stainless steel, cemented carbide, rubber lining, Monel alloy, valve body material
6. The occasions where the cast iron valve body is not suitable to include:
   1) Water vapor or wet gas with high water content;
   2) Flammable and explosive fluids;
   3) Where the ambient temperature is lower than -20℃;
   4) Compressed gas