- Jun 18, 2019
- Hits: 1998
The difference between single-seated and double-seated control valves is the number of plugs that close the flow path. A single-seated valve has one plug and one seat. It shuts off tightly, but the full pressure differential pushes on the plug, so it needs more actuator force. A double-seated valve has two plugs on one stem and two seats. The pressure forces on the two plugs largely cancel, so a small actuator can move a large valve, but the valve leaks more when it is closed. The two types are also called single-seat and double-seat control valves.
Both are globe-style valves and both are fitted with the same actuators. For how the actuator and positioner work on either type, see our pneumatic control valve guide.
| Single-seated | Double-seated | |
|---|---|---|
| Plugs and seats | One plug, one seat | Two plugs on one stem, two seats |
| Force on the plug | Unbalanced: the full differential acts across the port | Largely balanced: the two plugs oppose each other |
| Actuator needed | Larger for the same size and pressure | Smaller |
| Shutoff to ANSI/FCI 70-2 | Typically Class IV with a metal seat; Class VI with a soft seat | Typically Class II, at most Class III |
| Flow capacity for the same size | Lower | Higher |
| Guiding | Top-guided | Top-and-bottom guided |
| Behavior at small openings | Stable when the flow tends to open the plug | Can vibrate as the net force changes direction |
| Maintenance | One seat to lap | Two seats that must be matched |
| Typical use | Small and medium sizes where leakage matters | Large flows where some leakage is acceptable |
One plug moves against one seat ring. The plug is guided at the top, and the flow normally enters under the plug so that it tends to push the valve open.
Because nothing balances the plug, the actuator has to overcome the pressure differential multiplied by the port area, plus the seat load and packing friction. The force grows with the square of the port diameter:
| Port diameter | Differential pressure | Force on the plug |
|---|---|---|
| 50 mm | 10 bar | about 2.0 kN |
| 100 mm | 10 bar | about 7.9 kN |
That is why single-seated valves are found mostly in small and medium sizes, and why their allowable shutoff pressure falls as the size goes up. In return, one plug on one seat gives a single, well-defined sealing line, which is the tightest arrangement a globe valve offers.
Shinjo's ZJHP single-seated globe control valve is published in DN20 to DN350, PN16 to PN64 and Class 150 to 900. It is intended for duties with a small allowable leakage and a limited differential pressure. The electric version is the ZAZP.
Two plugs are fixed to one stem and close on two seat rings. The flow splits and passes through both ports. The differential pressure pushes one plug toward its seat and the other away from it, so the net force on the stem is small.
This has two benefits. A small actuator can operate a large valve at a high differential pressure, and two flow passages give more capacity than one for the same body size.
It also has two costs:
- Shutoff. Both plugs cannot land on their seats at exactly the same instant. Machining tolerances, and the different thermal expansion of the body and the plug assembly, leave one seat slightly open. Leakage is therefore higher than in a single-seated valve.
- Stability. The small net force on the plugs can change direction as the differential changes, which may cause vibration and noise at small openings.
Shinjo's ZMAN double-seated control valve covers DN40 to DN200 in Class 150 to 600, with a published leakage of Class II, at most Class III. The electric version is the ZAZN.
"Tight" is not a specification. ANSI/FCI 70-2 defines seat leakage classes, and each valve construction has a class it normally meets.
| Class | Maximum seat leakage | Typical construction |
|---|---|---|
| II | 0.5% of rated capacity | Double-seated valves; balanced single-seat valves with a piston-ring seal |
| III | 0.1% of rated capacity | The same types, built to a tighter standard |
| IV | 0.01% of rated capacity | Unbalanced single-seated valves with a metal seat |
| V | 0.0005 ml/min of water per inch of seat diameter per psi of differential | Metal-seated single-seated valves with high seat load |
| VI | A fixed number of bubbles per minute for each port size, tested with air | Soft-seated valves |
A class is a test result on a new valve under stated test conditions, not a lifetime guarantee. The test conditions and what each class permits are covered in Control Valve Selection: The 7 Numbers That Decide It. Seat materials are covered in What Is a Valve Seat and Seat Ring?
Many duties that once went to double-seated valves now go to cage-guided valves with a balanced plug. Holes through the plug equalize the pressure above and below it, so the actuator force is low, as in a double-seated valve. But there is only one seat, so the shutoff is better. Shinjo states that its metal-seated cage trim can meet Class IV to GB/T 4213; see the ZJHM cage-guided control valve.
The cage design has its own limit. The close clearance between plug and cage does not tolerate solids, so a double-seated valve is still the better choice for a slightly dirty fluid where Class II or III leakage is acceptable.
- Choose single-seated when tight shutoff matters, the valve is small or medium in size and the differential pressure is moderate.
- Choose double-seated when the flow is large or the differential is high, Class II or III leakage is acceptable and the fluid may carry some dirt.
- Choose cage-guided balanced when the differential is high, Class IV shutoff is needed and the fluid is clean.
- If a single-seated valve would need an impractically large actuator, look at a balanced design before stepping up the actuator.
Whichever type you choose, write the required shutoff class on the datasheet rather than the word "tight," and give the maximum differential pressure the valve must close against. The actuator is sized from that figure; see What Is a Pneumatic Diaphragm Actuator?
A single-seated valve leaks less. With a metal seat it typically meets ANSI/FCI 70-2 Class IV, and with a soft seat Class VI. A double-seated valve is typically Class II or III, because its two plugs cannot both seat perfectly at the same time.
The differential pressure pushes one plug open and the other closed. The two forces largely cancel, so the actuator only has to overcome the small difference between them, plus friction.
No. It is a throttling valve with Class II or III leakage. If the line must be isolated, add a separate isolation valve or use a single-seated or soft-seated design.
It is one way to balance the forces on the plug. The other is a single plug with balancing holes that slides inside a cage. Both reduce actuator force, but the cage-guided design has only one seat and shuts off more tightly.
For the same body size, a double-seated valve has the higher capacity, because the flow passes through two ports instead of one.
