When a valve fails in a steam system, the pressure is on to replace it quickly. In the heat of the moment, many buyers choose a replacement based purely on cost rather than functionality and reliability. Unfortunately, this is often where the real problems begin.
In steam systems, selecting the wrong valve technology can create long term operational problems that far outweigh any short-term savings. A frequent and expensive mistake in steam applications, particularly in Brewery and Process Plants, is fitting a Wedge Gate Valve (WGV), where a Parallel Slide Gate Valve (PSGV) should be used.
At first glance, both valves appear externally similar and are often simply described as ‘Gate Valve’. In reality, they behave very differently especially under working conditions.
Choosing the wrong one can lead to:
- Pressure lock-up
- Seized valves
- Excessive operating torque
- Unplanned plant downtime
- Premature valve failure
- Costly replacement and rework
Customer Case
When a nationally recognised chemical plant experienced serious operational issues, the problem was eventually traced back to the use of Wedge and Slab Gate Valves in an application more suited to Parallel Slide Valves; the wedge gate valves were unsuitable for the plant operating conditions. and under steam pressure, the wedging mechanism created significant operating difficulties and reliability concerns almost immediately.
As a result, the entire batch had to be replaced, effectively doubling the cost before the system could be stabilised. Blackhall supplied the correct Parallel Slide Gate Valves and the performance improved significantly once installed. This case highlights the operational and financial risks of specifying the wrong valve technology for critical duties. From this we can deduce that cutting cost upfront leads to a much greater cost later.
Parallel Slide vs. Wedge-gate: Why it matters
The two may look similar from the outside and share the same characteristic of being full bore through flow valves, but they work on completely different principles.
Wedge Gate Valve
A Wedge Gate Valve seals by mechanically forcing the wedge into the inclined seats creating a tight metal to metal seal. Under steam, this can be a disaster.
A Wedge Gate Valve:
- Requires constant axial force to drive the wedge into the gate and achieve a seal.
- Opening must overcome static pressure, mechanical wedging and component mass.
- If the temperature of the components rises on exposure to steam flow, the wedge will be driven hard into place by the expansion of the valve stem making it much more difficult to open and raising the possibility of component failure due to excessive load on opening.
Parallel Slide Gate Valve
A Parallel Slide Gate Valve uses two flat discs that rely on fluid pressure to seal against two parallel faces in the body. Under non-operational conditions, the two discs are held apart and in position by a spring; this maintains initial contact and is an aid to achieving initial sealing on first pressurisation.
- The sealing is pressure-assisted: the upstream disc lifts from its seat and allows line pressure into the body cavity; the pressure then forces the downstream disc against the downstream face and produces a tight seal assisted by the line pressures.
- Only one disc is under load during opening up to the point where the disc traverses the bore; this reduces the ‘breakout’ force and allows for the use of smaller actuators or gearboxes in the operating system.
- The parallel slide gate valve is a ‘position seated’ valve; the positions of the discs when fully closed and open are set by the design of the top end assembly. THERE IS NO CONTINUALLY APPLIED AXIAL LOAD and therefore no resulting ‘locking in’ action as in the wedge gate valve, and no danger of locking caused by expansion of components.
- The parallel slide gate valve operates with a self-cleaning action; debris from the fluid accumulating on the seat is wiped off during the opening and closing phases, whereas with a wedge gate valve, debris would tend to become embedded in the sealing face causing leakage problems over time.
Why Do Wedge Gate Valves Keep Getting Fitted?
Three reasons:
- They are generally cheaper initially.
- They are widely stocked, therefore readily available
- Many buyers are not fully aware of the differences, especially if the schematics or paperwork simply mandate a “gate valve.”
Equalising Ports and Bypass Lines: Why They Matter
Pressure Lock can be a problem equally with wedge gate valves and parallel slide valves, often being dependant on operating conditions changing over time or the design of the piping circuit itself.
The simplest form of equalisation/bypass is the straight loop; this is a straight connection between the inlet and outlet ports of the valve with a stop valve between the two points. This is the commonest type of bypass used on wedge gate valves and takes some of the load from the wedge by slightly lowering the upstream pressure and thus reduces the opening load. On steam service, it is often used (on PSGVs) to allow pre-heating of the downstream piping circuit, thus minimising thermal shock in addition to lowering the operating load.
The second most common type of equalisation is simply known as an equalising pipe. This comprises a pipe from the inter-gate body cavity to the inlet (high pressure) end of the valve. This ensures that the internal body cavity can never exceed the inlet pressure and allows the PSGV to operate in the intended mode. This equalisation is to guard against the expansion of cold condensate within the body cavity as working temperatures are reached. Under such circumstances ‘hydraulic locking’ can occur, which results in both discs being forced hard against their faces and increasing opening loads and stresses on the valve components.
It should be noted that fitting of an equalisation pipe makes the valve essentially uni-directional
If you need further advice on equalisation/bypass uses and advisability, please see our technical bulletin or contact our technical sales department.
Parallel Slide Gate Valves Advantages – Summary
With increased demands for reliability, efficiency and safety under high differential pressures, Parallel Slide Gate Valves offer several performance and sustainability advantages, such as:
- Reliable Operation Under Full Pressure: The independent disc design significantly reduces unseating force, making PSGVs ideal for emergency or startup conditions.
- Enhanced Safety: Lower operating torque reduces stress and failure risk. Engineers can operate the valve safely with less physical effort, reducing the risk of injury or incorrect operation.
- Thermal Cycling Tolerance: PSGVs can operate reliably with rapid temperature fluctuations without sticking or jamming.
- Maintenance Reduction: Self-cleaning seat action reduces wear which translates into longer life and therefore fewer service intervals.
- Sustainability and Long Asset Life: PSGVs are often repairable and designed for long-term service, helping reduce environmental impact. In contrast, Wedge-gate Valves are often inexpensive and treated as disposable commodity.
Key Takeaway’s
Installing a Wedge Gate Valve to your steam system when a Parallel Slide Gate Valve is better suited is a significant operational risk and will ultimately result in a shortened life or a costly replacement. Blackhall manufactures both WGVs and PSGVs as part of our Taylor Shaw range therefore our recommendations are based purely on engineering suitability, not product bias. Our advice is based solely on long-term reliability, safety and sustainability.
