Most practitioners have been in this situation at some time or other: a control valve has been prepared according to the process data, but, by the time it is ready to be installed, the requirements of the process have changed. The start-up deadline is fast approaching and the costs have to be controlled within budget. When something like this happens, quick help is vital – and this is where the flexibility of Spira-Trol valves is invaluable.
Klaus Rümler
Even though many users show a lot of respect for them, modern control valves are actually quite simple devices. What is needed is the highest possible performance at minimum cost – a control valve that can respond quickly, is flexible to use and is guaranteed to perform excellently both during start-up and in the long term. Spirax-Sarco has therefore completely redesigned the Spira-Trol valve to include self-aligning, clamp-in-place internals (figure 1). All inside parts can be replaced using a standard spanner and even inexperienced staff can carry out this work themselves easily and with excellent results.
If the process load is not what it seems at start-up, the Kvs value of the control valve must be changed. With many traditional valves this is either not possible or, at best, very time-consuming. With Spira-Trol the seat retainer and valve seat can be removed by hand with the valve open. The valve stem and plug can then be conveniently pulled out of the bonnet – no special tools are necessary at any stage.
Figure 2 shows the difference: on the right is a much lower Kvs value with the exchanged components on the left. All other valve components have remained unchanged.
After exchanging the valve seat gasket, the new seat and seat retainer are assembled in the valve by hand. The new valve stem and plug are carefully inserted into the bonnet through the gland seal. The bonnet gasket is replaced and the bonnet fitted onto the valve body. The valve seat must now be centred by pushing down the stem so that the plug presses against the seat. The job is finished as soon as the bolts have been tightened. The original problem – an incorrect valve Kvs value – is eliminated within minutes. In addition to being very simple and quick, the whole operation requires no special tools and to save even more time the valve body can stay inside the pipeline.
Soft seal replacement
When controlling a hot medium, it is common to use control valves with a metal-to-metal seal. However, metal sealing means that the tightness of the valve is never absolute and a small flow inevitably passes across the seat. This shouldn’t normally be a problem in steam plants providing a few basic rules are considered when the plant is designed and the equipment installed. In some applications, however – steam included – there are cases where a high degree of tightness is essential. This can only be achieved with a soft seal arrangement.
The disadvantage of soft seals is that abrasion in continuous operation and at high temperatures means they have to be replaced from time to time.
The replacement method is similar to that for changing the Kvs value: open the valve, exchange the components, replace the gaskets, centre the components and tighten the bolts. Spira-Trol provides an additional cost saving because the seal is located in the seat, so that it is not necessary to replace an expensive plug at the same time.
Reduced noise
The rate of flow in pipelines is low, typically 1 to 2 m/s for water, 25 m/s for steam or 40 m/s for gases. By contrast, the flow velocity in a control valve is very much higher. In some cases, Mach 1 may be reached. Depending on the mass flow and velocity, undesirable noise can be produced that is liable to damage the hearing of the operator and reduce the life of the valve.
There are a number of points to consider when tackling the amount of noise generated by a control valve. Firstly, the valve capacity should be correct. Another parameter is the size of the valve body – it is usually selected according to the stream flow in the outlet flange. A valve body that is too small is sometimes chosen for cost reasons. If everything else fails – a frequent dilemma in practice – a special perforated cage can be used to limit the generated noise. As Spira-Trol components have a simple clamp-in-place design, this cage can be fitted on site as described above (figure 3). A further important aspect is the design of the valve body, which in the case of Spira-Trol has been optimised with the help of a finite element analysis. The velocity in the body – and therefore the generated noise – is lower as a result and erosion is also reduced. The design additionally prevents the formation of jet streams, which can cause premature failure of the valve body wall.
A technical dilemma
The gland seal has to ensure there is no leakage from the valve stem. The most obvious – though not the best – approach would be for the gland seal to fit as tightly as possible. The pressure from the gland seal needs to be low to enable the valve stem to move easily; light pressure ensures good control quality and good repeatability. If the gland seal is fastened too tightly, the electric or pneumatic actuator will need a higher force to achieve even a small movement. This makes the solution more expensive and the higher force also means a risk of overshooting, i.e. there are increased variations in control due to hysteresis effects.
A gland seal is potentially exposed to harsh conditions from the operating requirements of the application, the process fluid and the environment, all of which shorten its life. For example:
- Dirty operating conditions, i.e. either dust or dirt from outside or solid particles from the medium may enter the stem seal.
- High temperatures can reduce the seal life.
- Frequent movement of the stem results in greater wear.
To minimise the impact of these conditions, Spira-Trol gland seals are equipped with a wiper at the top and bottom, which removes any particles from the valve stem and stops them entering the seal. The outcome is a much longer life and less maintenance.
The gland seal is also spring-loaded internally, so that the seal automatically adjusts to wear during operation. This has a positive influence on the valve’s maintenance requirement. Although the gland seal will have to be replaced in time, when this need arises, the procedure is very quick and easy and requires no special tools.
Danger from water hammer
Water hammer often occurs in poorly drained steam systems; water droplets collect and are propelled along the pipeline at high speed. If they hit the control valve internals, there is a possibility of damage and the valve stem may be bent. If the valve is harmed in this way, the following effect will be noticeable:
- The valve is unable to close properly and more flow passes across the seat when it is in the closed position.
- The valve stem seal life is reduced because the stem wears down one side of the seal and distorts its shape.
To protect against damage from water hammer, the valve stem has two guides positioned at very large intervals along the stem. This ensures excellent alignment of the stem through the seal and onto the valve seat, and results in quiet running similar to the long wheelbase of a luxury car.
Even if the control valve is damaged by water hammer, the valve body and bonnet can continue to be used – only the gland seal and valve stem need to be replaced.
Different types
Spira-Trol valves are available in all popular materials: cast iron, GGG 40.3, carbon steel and stainless steel. The most common dimensions are DN 15 to DN 100, with pressures from PN 16 to PN 40. Depending on the application, there are many options to choose from, including reduced Kvs, soft or metal sealing, low-noise trim and a PTFE, graphite or bellows gland seal. DIN or ANSI-compliant types with identical internal components can be supplied for international customers.
cpp 431
More about Spira-Trol
Steam Tables
Share:
