Steam quality plays an important role in determining the quality of final product. At the same time, steam quality is also important to ensure smooth working of the plant and operational efficiency. Steam quality should be monitored on a regular basis and proper actions should be taken to ensure that the process is getting the correct quality of steam. There are multiple factors which determine the steam quality. The steam which is used should meet following criteria:
1. Correct quantity
The steam which is available at the process should be in correct quantity to ensure the required heat flow rate. Improper flowrate could spoil the product and also drop the rate of production. Pipes should be correctly sized to get the required flowrate; also the steam loads should be correctly calculated. Flowrate is directly proportional to the sizing of the pipe. Flowrates are measured using flowmeters. Various flowmeters like orifice, variable area, spring loaded, vortex etc. are used depending upon application.
2. Correct Temperature and pressure of steam:
The steam should have correct temperature and pressure required for particular application. Incorrect temperature and pressure will affect the performance of the plant. Desired sizing of pipework and pipeline ancillaries will achieve correct pressure
. In some cases due to presence of air and incondensable gases; at required pressure the relative temperature is not achieved. Pressure reducing stations are total mechanical and instrumental system which is mainly used for reducing the pressure of the fluid. Control valves are used to reduce the pressure of the fluid; this is done by restricting the path of the flow. Temperature control valves receive the signal from RTD sensor. Based on which the control valve is throttled to achieve the desired temperature. This avoids the overshoot of temperature ensuring better product quality and productivity.
3. Free from air and incondensable gases:
At the start-up of the plant, air is present in the supply pipes and equipment. Even though at the last time of use the system is filled with pure steam, at the time of shutdown condensate is formed and air would be drawn in by the resultant vacuum. The air across the heat transfer surface forms an insulating layer, as it is bad conductor of heat.
Air carries no useful heat so if it mixes with the steam, the heat content of the mixture is reduced and its temperature is lowered. When the steam enters the system, the nature of the steam is to push the air ahead of it so that it collects at some point most remote from the inlet or to the drain point. Hence steam traps are fitted to the drain points with sufficient air venting capacities. Automatic air vents should be fitted to all remote points. If there is turbulence, the steam and air will mix and the air will be carried to the heat transfer surface. This will cause the temperature to be lower than required. Air causes problems like cold spots on heating surface, distortion and stressing of the equipment. It is also the root cause of the corrosion.
Steam remains today the prime carrier of heat for the process industries, from food and dairy to biotechnology and pharmaceuticals. It is used for the process heating and working environment as well as for sterilization and biopharmaceutical industry. Normal factory or plant steam may contain small quantities of boiler feed water chemicals and traces of pipe scale and other debris which may not be acceptable in the production of the injectable or parenteral solutions, which are always sterile. In the production of these parenteral solutions, quality and purity of steam is of utmost importance, and special care needs to be taken for the generation of steam. Layers of scales found on the pipe walls are due to formation of rust in old system and carbon deposits. These fragments will have effect of increasing rate of erosion in bend pipes and the orifice of traps and valves. To avoid this, strainers should be installed in upstream line of steam trap, flowmeter, pressure reducing valves and control valves. Fig 6.Strainer Steam flows through the inlet of strainer and then from perforated screen to the outlet. The dirt in steam is entrapped in the screen which can be removed by opening the cap and then cleaning the screen.
As we know that presence of water droplets in the steam reduces the actual enthalpy of evaporation, and also leads to the formation of scale on the pipe walls and heat transfer surface. Hence the steam reaching the plant is relatively wet. Droplets in steam also cause barrier to heat transfer process. These droplets when build along the steam line cause problem like water hammering. The slug of water will be carried at the steam velocity along the pipe work. Due to impact of slug of water and obstruction, noise and vibration occur. A moisture separator in the steam line removes droplets entrained in the steam flow, and also any condensate that has gravitated to the bottom of the pipe. The steam is forced to change the direction several times as it flows through the body. Baffles are designed so as to create obstacle for heavier droplets, while the lighter dry steam is allowed to flow freely through the separator. These droplets are collected at the bottom of the separator.