Steam is normally produced in industrial boilers at high pressures and temperatures - since in this superheated condition it carries more energy. This results in more efficient production of power in a turbine. The high pressure also allows the steam to be transported around the plant in smaller lines with less heat loss. Typical boilers in sugar mills work at 3 to 4 MPa (30 to 40 bar) at up to 420°C. In the power industry pressures in excess of 20 MPa (200 bar) and temperatures up to 550°C are not uncommon.
Both the pressure and the temperature have to be reduced for steam use in a process plant. This is partly so that the process equipment does not need to be rated for the high pressures and temperatures but also because heat transfer is much more efficient when the steam is saturated.
The danger of reducing the pressure too quickly is that very high velocities are generated that cause wear inside the valve and very high noise levels. The solution is to use a special valve that is engineered to solve the complex requirement of both pressure reduction and noise control. An energy dissipating disk stack is one of the best methods to achieve this. In this special trim the pressure is dropped across many stages, controlling the velocity at low levels.
The disk stack has a major advantage over most other trim designs in that it combines the concept of many restrictions in series (along each passageway) with multiple paths in parallel (several passageways in each disk and many disks in a stack). This enables the valve to handle low flow rates as well as the maximum flows. Many designs only have restrictions in series and so can handle the high flow rates but suffer severe damage at low openings.
The Mitech energy-dissipating disk stack trim fits into its range of standard globe valves. Often with high-pressure drop applications there is a major difference in the size of the upstream and downstream pipework. For example it would not be unreasonable for the upstream pipe to be 150 mm and for this to increase to 500 mm downstream. In this case an angle style of valve can further reduce both the overall size of the valve and the cost. The flow will be under the plug through the disk stack and out through the larger side port. An additional saving is that the whole valve does not need to be rated for the upstream conditions. The inlet porting can be designed to ANSI 1500# whereas the body of the valve can be rated for ANSI 300# as an example - provided of course that there is a suitable pressure relief valve installed in the downstream pipework.
One or more diffuser plates may be installed downstream of the valve to create backpressure to enable a smaller valve to be used. Typically this relatively cheap device can halve the size of the valve while maintaining the same velocity constraints. When this means using a 200 mm instead of a 400 mm valve the cost savings can run into several hundred thousand rands.
Reducing the pressure of the superheated steam does very little to reduce the temperature - in fact there are some conditions where a reduction of pressure can actually increase the temperature.
To reduce the temperature of the steam, a device is utilised that adds water to the superheated steam. To vaporise the water a large amount of energy is required to overcome the latent heat. The high temperature steam provides this. The result is an increased flow of steam at a reduced temperature. By varying the amount of water added, the temperature can be controlled.
If the temperature is lowered to within 10°C of that of saturated steam, at the actual pressure involved, this unit is called a desuperheater - if the required temperature is such that the steam is still superheated then the device is called an attemperator.
The secret of success for any desuperheater is to ensure that the water atomises into a fine spray so that the water droplets can be absorbed by the steam. If this does not happen the result is poor temperature control and cracked pipework downstream.
There are three basic principles used in the various designs of desuperheaters. The first requires a supply of high-pressure water to be available, at say, 2 MPa (20 bar) above the steam pressure. In this category are the mechanical nozzle systems. With high pressure available at a nozzle it is relatively easy to cause the water to atomise but the problem occurs as the flow to the nozzle arrangement is throttled back in the spraywater control valve. When this happens the pressure at the nozzle drops and so the water is not atomised as effectively. The turndown on a standard nozzle is no more than 2:1 - much less than is usually required. Some designs make use of many nozzles of different sizes and a mechanism to open and close these nozzles to control the flow - this ensures that the full water pressure is available at the working nozzles all the time. Very high turndowns up to 60:1 are achievable. The disadvantage of these units is the initial expense and the fact that there are moving parts in the steam flow that leads to maintenance requirements and high total cost of ownership.
Mitech makes a spillback desuperheater that takes the high pressure water to a single or multinozzle arrangement and then controls the amount of water that flows back from the unit to a condensate return point. This gives good control and turndown ratios of 15:1. The main disadvantage is the need for a return line and the consequent waste of energy. These desuperheaters are particularly good for small units where only a few hundred kilograms of water need to be added each hour.
The second group of desuperheaters are known as steam atomising or steam assisted. These require a source of high-pressure steam to be available which is one reason why they are popular in the sugar industry when used as part of a pressure reducing and desuperheating station. With this design a small part of the total steam flow is bled off from the HP system and fed to a nozzle arrangement where low-pressure water is injected into the high velocity steam flow. The velocity of the steam assists with the atomisation of the water. This type of unit offers high turndowns of up to 25:1 but has no moving parts in the steam flow - a major advantage when it comes to long life. Mitech has recently introduced its range of steam atomising desuperheaters for use in these applications.
The third form of desuperheater is the type that creates a low-pressure region in the main steam flow by means of a venturi and injects low-pressure water into this area where the water is more likely to vaporise. A variation of this theme is the combination valve-desuperheater, where the water is fed though passageways in the valve body and then injected into the steam flow just downstream of the seat ring at the vena contracta where the pressure is low. This has the disadvantage that noise control is not possible due to the fact that low noise valves are designed to keep velocities low by ensuring that the pressure does not drop too low. Another disadvantage is that the injection of cold water into a hot valve can cause thermal cracking of the body and other valve components.
Mitech
(011) 462 2160
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