Trainer A.R.KANADE [email protected]Why Reduce Pressure? There are a number of very good reasons for reducing steam pressure: Steam boilers are usually designed to work at high pressures. Working them at lower pressures can result in carryover of water Steam at high pressure has a relatively small volume which means that a greater weight can be carried by a pipe of a given size. It is preferable to distribute steam at high pressure and reduce it at the point of usage Steam pressure may be reduced to save energy. Steam at lower pressures has higher latent heat. Reduced pressure of steam also leads to reduced heat loss and lower flash steam formation from open vents etc. Since the pressure and temperature of steam are related, controlling the pressure enables us to control the temperature in the heating process Pressures must be reduced so that they are within the rated safety limits In plants where steam usage takes place at many different pressures, pressure reduction allows generation of steam at a single high pressure and subsequent reduction to the desired pressure at the point of usage
There are a number of very good reasons for reducing steam pressure: Steam boilers are usually designed to work at high pressures. Working them at lower pressures can result in carryover of water - PowerPoint PPT Presentation
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There are a number of very good reasons for reducing steam pressure: Steam boilers are usually designed to work at high pressures. Working them
at lower pressures can result in carryover of water Steam at high pressure has a relatively small volume which means that a
greater weight can be carried by a pipe of a given size. It is preferable to distribute steam at high pressure and reduce it at the point of usage
Steam pressure may be reduced to save energy. Steam at lower pressures has higher latent heat. Reduced pressure of steam also leads to reduced heat loss and lower flash steam formation from open vents etc.
Since the pressure and temperature of steam are related, controlling the pressure enables us to control the temperature in the heating process
Pressures must be reduced so that they are within the rated safety limits In plants where steam usage takes place at many different pressures,
pressure reduction allows generation of steam at a single high pressure and subsequent reduction to the desired pressure at the point of usage
Operating Principle Downstream pressure set by adjusting
screw (A) This compresses the pressure
adjustment spring (B) onto the pilot diaphragm (C), opening the pilot valve (D)
Control steam passes through pipe (E) into the main diaphragm chamber and also through the control orifice (F)
As the flow through the pilot valve exceeds flow through the control orifice, the pressure under the main diaphragm (G) increases, opening main valve (H) against its return spring (I) and the supply pressure
Operating Principle... (Cont.) Steam flow through the main valve
increases the downstream pressure, which acts through pressure control pipe (J) onto the underside of the pilot diaphragm
When the upward pressure on the diaphragm balances the downward force of the spring (B), the pilot valve throttles
The control pressure it maintains under the main diaphragm positions the main valve to pass just enough steam to achieve the desired downstream pressure
An increase in the downstream pressure caused by a reduction in the steam load will reposition the pilot valve and reduce the control steam flow into pipe (E).
By understanding the Droop Characteristics we can: Select the most appropriate type of valve - pilot / self acting Select a set pressure for the safety valve that will prevent premature operation Understand the quality of control that can be expected under varying loads
Droop: When meeting a steady steam demand, any reducing valve will open just
enough to pass the desired amount of steam and maintain the reduced pressure
The downstream pressure will fall if the steam demand increases The reducing valve will sense the falling pressure and reposition itself so that it
will again pass enough steam to meet the increased load Since the valve must remain in this position if it is to continue to pass the
desired flowrate, the downstream pressure must be controlled at the lower level
The change in downstream pressure required to open the valve further is referred to as DROOP
Maintain excellent accuracy Can take upstream pressure variations of 20% Diaphragms do not stick like pistons Diaphragms made of SS: not highly stressed Inbuilt strainer Fluent movement of main valve Operates on dead end service only Easy trouble shooting Additional internal piping for balancing pressure Main valve hardened to 50 RC Pressure spring easily changeable Pressure turndown ratio 15-12:1 Pilot valve assembly identical for all sizes
Steam or air enters through the inlet connection, passes through the strainer screen (1) and then through the main valve seat (2) to the outlet. The downstream pressure acts on the inside of the bellows through three ports (3).
The position of the main valve (4) is determined by the balance of the forces acting on the bellows (5). The force exerted by the control spring (6) which is trying to open the valve is opposed by the return spring (7) plus the downstream pressure inside the bellows.
Increasing the compression of the control spring by turning the adjustment know (8) forces the main valve open allowing more steam or air to pass through to the downstream side. The reduced pressure must now build up sufficient pressure inside the bellows to close the valve. Decreasing the control spring compression has the opposite effect.
Consider a 200 litre open tank in which process liquor is being maintained at 85C, working pressure 3.5 bar and steam consumption max. 70 Kg/hr
Recommended: 1/2” TR 121 Without automatic control temperature could go upto 95C,
an unnecessary increase of 10C. This means about 2000 Kcal extra heat consumed by the liquor and 500 Kcal by the vessel. This means about 4.5 Kg of steam
This extra consumption could occur every 10 minutes. By the use of a TR 121, this can be eliminated
SAVINGS = 4.5 Kg steam / 10 minutes= 27 Kg/hr= 130 Tons/Yr (4800 working hours)= Rs. 19,500 yearly= 3 MONTH PAYBACK PERIOD
Assuming that the PRS is working under the following conditions Inlet pressure 10.5 bar Outlet Pressure 3.5 bar Flow 1000 Kg/hrLatent heat available @ 10.5 bar - 475 kcal/kg, @ 3.5 bar - 510 kcal/kg By reducing pressure a gain of 35 kcal/kg is achieved For 1000 Kg/hr flow of steam - 35,000 kcal/hrIn terms of Rs. saved For furnace oil with a calorific value of 10,000 kcal/kg and cost of Rs.
3,800/ton this means a saving of Rs. 14 per hour If installed in a plant running 16 hr/day, 26 days/month for 12 months the
savings are Rs. 70,000A similar PRS would cost Rs. 50,000