5.1 Introduction
If steam condenses in a pipe, the condensate must be discharged immediately it is formed for the following reasons:
- Condensate in steam pipes has disastrous consequences for turbine blades.
- Condensate in steam pipes leads to heat loss.
- Condensate in pipes results in corrosion.
- Condensate in steam pipes leads to erosion, for example at pipe bends.
- Condensate is a cause of water hammer in steam pipes.
Drainage is essential to remove condensate from pipes. This applies not only when a steam pipe system is actually operating but also during start-up. This chapter describes suitable drainage measures as well as the valves required to implement them. Further information about the consequences of water hammer can be found in the Chapter 6.0 Condensate Management.
5.2 Drainage of steam pipes
The technical design of steam pipes requires a continuous downward gradient (approx. 0.5 %) in the direction of flow. The drain points in a properly insulated pipe should be a maximum of 75 metres apart. An additional drain point must be provided before the start of each rising pipe section. It is not sufficient to discharge the condensate simply by installing a tapping in the main pipe. A DN 200 steam pipe with a DN 25 tapping cannot be drained because most of the condensate is entrained beyond the tap.
A drain pocket represents the only way to drain a steam pipe reliably. Fig. 5-1 shows a drain pocket in a straight pipe, while Fig. 5-2 depicts a rising pipe bend.
Fig. 5-1: Drainage in a straight pipe
Fig. 5-2: Drainage in a bend
The dimensions of the drain pocket should be determined as shown in Fig. 5-3.
Fig. 5-3: Dimensions of a drain pocket
The steam trap must be connected approximately 50 mm above the bottom and mounted on the side of the pocket. The bottom of the drain pocket then acts as a strainer that protects the steam trap against debris in the pipe. Drain pockets with a continuous blowdown device have the advantage that significant amounts of debris can be removed very easily. For simple drainage (Fig. 5-4), the steam trap should be mounted in the branch and the manual drain point in the vertical pipe section. In this case, the straight pipe acts as a strainer.
Fig. 5-4: Simple drainage with strainer
If the end of a pipe needs to be drained, a pipe bend is used as the drain port (Fig. 5-5). Instead of a pipe bend, the steam trap can also be mounted directly to the end flange. It is important to ensure that the trap is connected level with the bottom of the pipe rather than centrally.
Fig. 5-5: Drainage at the end of a pipe
Particularly in critical equipment, separators are often used to prevent condensate from entering the equipment. Steam jet compressors and vacuum pumps are especially sensitive to entrained condensate drops. Fig. 5-6 shows a cyclone separator. The water drops are spun against the wall by the centrifugal force and collect at the lowest point, where the steam trap is located.
Fig. 5-6: Cyclone separator
Fig. 5-7 shows several typical drain points.
Fig. 5-7: Typical drain points
The most important drain points are as follows:
- Drain point upstream of the control valve
- Drain point on a steam header
- Drain point in a steam pipe to the steam turbine
- Drain point downstream of an injection valve for steam temperature control
- Drain point at the end of a pipe
- Drain point upstream of a rising pipe bend
- Drain point upstream of a steam jet system
Drain points are essential in underpasses or at the transition from a high pipe bridge down to ground level and back up again to another pipe bridge. Eccentric pipe reducers are often installed at changes in pipe diameter. The underside of the pipe and the underside of the eccentric reducer must be located in the same plane to prevent dams from forming (Fig. 5-8).
Fig. 5-8: Connection of an eccentric pipe section
5.3 Putting steam pipes into service
Considerable care must be taken when steam pipes are put into service. The large mass of cold steel requires a relatively long time to reach saturated steam temperature. One metre of DN 150 pipe corresponds to a mass of 15 kg needing to be heated. When selecting steam traps, you should remember that this maximum condensate flow rate also has to be drained off at low pressures.
All drain points must initially be brought on-line by opening the globe valves upstream and downstream of the steam traps. Next, the main valve on the steam header must be slowly opened to the 'cracked open' position. Wait until condensate emerges with steam at each drain point. The pressure in the pipe can then be gradually increased by successively closing all the drain points.
When all the drain points are closed, any condensate that forms is only able to escape through the steam traps. The traps must remain on-line for some time before it is safe to assume that all condensate has been completely drained from the pipe.
The system is now at operating temperature.
5.4 Removing steam pipes from service
When a system of steam pipes is removed from service, it is important to remember that the steam in the pipes condenses. If the steam pipe is not ventilated, a vacuum will form inside it. Low-pressure steam pipes with thin walls and a large diameter have a tendency to become deformed as a result of vacuum. Completely emptying a product vessels (e.g. chemical containers) is another potentially for a vacuum situation.
5.5 Start-up drain valves
The amount of condensate that builds up during heating is 8 to 10 times more on average than the quantity that collects in a properly insulated pipe during normal service. If a steam trap is rated for the amount of condensate that occurs during the heating phase, it will be 10 times larger than the size that is required for normal operation. Conversely, if a trap is designed for the amount of condensate that collects during normal operation, it will be much too small to drain off the condensate that builds up as a result of heating. An automatic startup drain valve provides the perfect solution to this dilemma (Fig. 5-9). This valve combines a steam pressure-controlled function with a temperature-controlled function. The temperature control part is taken care of by the bimetallic assembly.
When the steam pipe is put into service, a compression spring holds the valve open to enable air and cold condensate to be drained off up to a closing pressure of 1.5 bar. If the pressure is greater than 1.5 bar, the drain valve remains closed. When the steam pipe is removed from service, the valve opens as soon as the pressure falls below 1.5 bar. The pipe is then completely drained without a vacuum forming.
Fig. 5-9: Automatic start-up drain valve, ARI Type CONA® 665
In production facilities where the steam boiler plants are shut down at weekends, automatic start-up drain valve enable the steam pipes to be put into service faster and eliminates water hammer. Using the boiler operator's experience, the steam boiler may be maintained at a pressure slightly less than the 1.5 bar closing pressure of the valve. The valve opens and the steam flashes. The large amount of condensate that has collected is drained off without the need for cyclic inspections and without having to open or close any bypass valves. Start-up drain valves help increase productivity by reducing the time to start up and shut down a plant. Production outages due to water hammer or defective components are a thing of the past.
Start-up drain valves are also suitable for condensate drainage if the apparatus is started up at such a low steam pressure that the condensate cannot be returned to the closed condensate system.
When a cold plant is put into service, the condensate is initially drained off by means of the drain valves. The steam trap connected in parallel does not begin to discharge to the condensate system until the steam pressure has risen sufficiently.
5.6 How to prevent freezing
Room air heaters that draw in fresh air have a tendency to freeze if the outdoor temperature falls below 0 °C and the plant is not under positive steam pressure. A small amount of condensate always remains in the heater batteries when the plant is removed from service. If the start-up drain valve is installed underneath the room air heater, it can drain off residual condensate when the heater is shut down.
5.7 Draining remote steam pipes
In many cases, condensate from remote steam pipes is not returned to the boiler house, for example because there is no condensate pipe. If one is installed, it is often completely filled with cold condensate, so that newly supplied condensate is able to flash.
If there is no cost-effective way to return the condensate, the best alternative is to connect the steam trap via a simple sandpit containing gravel by means of an open standpipe. The flash steam is then discharged into the open and the condensate drains into the gravel. Freezing is reliably prevented, especially in winter.
5.8 Steam traps for pipe drainage
Selecting the right steam trap is not always easy. Each type of trap has its own specific advantages, but it can also have disadvantages if used incorrectly. The choice is mainly determined by the operating mode and the situation in the field.
Drainage with thermostatic steam traps
Thermostatic steam traps guarantee good air venting during start-up. As long as the condensate temperature is lower than the saturated steam temperature, the trap remains fully open and condensate and air are discharged quickly. Thermostatic steam traps adapt automatically to increases in pressure or temperature.
They close again as soon as the condensate temperature approaches the saturated steam temperature. The trap opens when the condensate cools down to a temperature approximately 10 ºC lower than the saturated steam temperature. Condensate backs up while the steam trap is closed. It is advisable not to insulate the globe valve upstream of the steam trap or the steam trap itself, in order to avoid unnecessarily prolonging the time between the trap's "closed" and "open" states. Many pipes are uninsulated as much as a metre upstream of the steam trap.
Drainage with inverted bucket steam traps
Inverted bucket steam traps work independently of the condensate temperature. Drainage of the collected condensate is controlled according to the level without backing-up. However, inverted bucket traps are relatively slow when it comes to venting air, so that the start-up process may be delayed. If superheated steam reaches the trap, the water seal could boil empty. If it loses its water seal and remains open, steam is lost. This can happen in practice if the connecting line between the steam pipe and the steam trap is too short or if the pressure drops in the steam pipe. It is advisable not to insulate the steam trap or the upstream globe valve.
Drainage with thermodynamic steam traps
Thermodynamic steam traps work independently of the condensate temperature. The valve disc inside the trap closes before saturated steam has a chance to exit at high velocity. A steam cushion forms above the valve disc, forcing it down and closing the steam trap. When this steam cushion condenses, the valve disc can no longer be closed against system pressure. It sometimes happens when a steam pipe is put into service that air from the pipe is bound over the valve disc. The trapped air prevents the steam trap from opening correctly. Vent holes drilled in the disc to permit air venting can result in leaking steam.
Drainage with ball float steam traps
Ball float steam traps work independently of the condensate temperature. Drainage of the collected condensate is continuous and controlled according to the water level inside the trap. Good ball float traps are fitted with an air vent.
If superheated steam reaches the trap, the chamber can boil dry. In contrast to the inverted bucket principle, however, no steam is lost by the ball float trap because the float sinks to the bottom of the housing and the steam trap valve remains closed.
Drainage with fixed orifice steam traps
Fixed orifice steam traps (also known as venturi traps) are designed to allow a precisely calculated amount of condensate to flow under constant, defined pressure conditions. The diameter of the drain orifice is exactly rated for this flow.
Since the diameter is constant, the orifice trap is unable to respond to variations in the operating parameters. Steam may be lost as a result. Fixed orifice steam traps cannot react to the significant differences in flow rate between start-up and normal operation because they lack the variable valve system of other trap types. The use of these traps to drain steam pipes is not recommended for this reason. The operating principle of the various steam trap types is described in detail in Chapter 7.0 Steam Traps.