Q&A of valve type selection (part two)

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Posted: 04/08/2019 14:54:05 Hits: 61

What are the characteristics and applications of straight-through single-seat control valves?
(1) The discharge flow is small, the sealing performance of this kind of valve can be assured since it has only one valve core. The standard discharge flow is 0.01% KV and can be further designed as a shut-off valve.
(2) The allowable pressure difference is small because the unbalanced force is large. The pressure difference of a DN100 valve is only 120KPa.
(3) The circulation capacity is small. The KV value of a DN100 valve is only 120. It is often used in applications where the amount of leakage is small and the pressure difference is not large.

What are the characteristics and applications of the straight-through double-seat control valves?
(1) The allowable pressure difference is large because it can offset many imbalance forces. The pressure difference valve of a DN100 valve is 280 KPa.
(2) Large circulation capacity, The KV valve of a DN100 valve is 160.
(3) The amount of leakage is large because two valve cores cannot be sealed at the same time. The double-seat valve has a standard discharge of 0.1% KV, which is 10 times that of a single-seat valve. The straight-through double-seat control valve is mainly used in applications where the high-pressure difference and the leakage are not critical.

Why is the anti-blocking performance of the straight stroke control valve poor, while that of the angular stroke valve is good?
The straight-stroke valve core is vertically throttled, and the medium flows horizontally in and out, and the flow passage in the valve chamber is inevitably turned and inverted, making the flow path of the valve quite complicated. Thus, there are many dead zones that provide space for the precipitation of the medium, which in the long run causes blockages.

The direction of the angular stroke valve throttling is the horizontal direction. The medium flows horizontally, which makes it easy to take away the impurities. At the same time, the flow path is simple, and the space for the medium to precipitate is also small, so the anti-blocking performance of the angular stroke valve is quite good.

Under what circumstances is a valve positioner needed?
(1) Situations where the friction is large and precise positioning is required, such as high temperature, low-temperature control valves or control valves using flexible graphite packing.
(2) Situations where the response speed of the control valve needs to be increased, like adjustment systems for parameters such as temperature, liquid level, etc.
(3) Situations where the output force and cutting force of the actuator need to be increased, such as a single seat valve with DN larger than or equal to 25 and a double seat valve with DN larger than 100; when the pressure drop across the valve is larger than 1 MPa or the inlet pressure larger than 10 MPa.
(4) When the split-range adjustment system and the control valve are operated, it is sometimes necessary to change the air opening and air-closing forms.
(5) Where it is necessary to change the flow characteristics of the control valve.

What are the steps to determine the valve diameter?
(1) Determine the calculated flow rate (max and min).
(2) Determine the calculated pressure difference - select the resistance ratio value according to the characteristics of the system, and then determine the pressure difference (valve is fully open).
(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.
(5) Check the opening degree.
(6) Actual adjustable ratio.
(7) Determine the valve diameter - if unqualified, re-select KV value, and then verify.

Why can't the sleeve valve replace the single or double seat valves?
Single-seat valves, double-seat valves, and sleeve valves are equally used today. This is because the sleeve valve only improves the throttling form, stability and reduce the maintenance difficulty compared with the single seat valve, but its weight, anti-blocking and leakage indicators are consistent with single and double seat valves.

Why is it better to choose hard seal with shut-off valve?
The shut-off valve requires that the leakage is as low as possible. Even though the leakage of the soft-sealed valve is the lowest, and the cutting effect is, of course, good, but it is not wear-resistant and has poor reliability. From the standard of small leakage and reliable sealing performance, the soft seal is not as good as the hard seal.

Why is the valve stem of the straight stroke control valve relatively thin?
It involves a simple mechanical principle - The larger the sliding friction, the lower the rolling friction. The valve stem of the straight stroke valve moves up and down, and the packing is slightly pressed tightly, which will tightly seal the valve stem and produce a large backlash. For this reason, the valve stem is designed to be very small, and the packing is also commonly used with PTFE packing that has a small friction coefficient to reduce the backlash, but the problem is that the valve stem is thin and easy to bend, and the service life of the packing is short. The best way to solve this problem is to use the valve stem of the discharging valve, that is, the angular stroke type regulating valve whose valve stem is 2 to 3 times thicker than the straight stroke valve stem, and choose graphite packing with long service life.

Next: The Common Types of Check Valves

Previous: Q&A of valve type selection (part one)

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Teresa

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