Why Reduce Pressure?
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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
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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 - PowerPoint PPT Presentation
Transcript of Why Reduce Pressure?
Trainer [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
Trainer [email protected] Pilot Operated Reducing Valves
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
Trainer [email protected] Pilot Operated Reducing Valves
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).
Trainer [email protected] Where To Use?
BRV (Direct Bellows Action) Small loads On/Off application Low maintenance Compact design Low cost
DP (Pilot Operated) Small to medium loads High control accuracy Wide product range variations Ideal close to process control
DRV (Direct Diaphragm Action) Medium to large loads Simple operation Robust design Mains pressure reduction High pressure turndown application
Trainer [email protected] Droop Characteristics
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
Trainer [email protected] Amount of Droop
If valve is set on no load: DP17 / DP143 0.2 bar BRV 1/2” 20% of no load pressure 3/4” 25% of no load pressure 1” 30% of no load pressure
If valve is set on maximum load: DP17 / DP143 0.2 bar BRV Pressure Increase = Set Pressure / (1 - Droop %)
If load increases the control pressure will decreaseIf load decreases the control pressure will increase
Trainer [email protected] Features of Spirax PRVs
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
Trainer [email protected] DP17: Salient Features
The control of downstream pressure is extremely accurate
The valve can accept an upstream variation of upto 20% with no effect on the downstream pressure
The valve will shut tight on dead end service If the correct pressure adjustment spring is used
and with correct installation, the valve will control 0.035 bar of the set pressure
This valve can be used for compressed air service with a soft seating arrangement
When the valve pulsates from wide open to wide shut, diaphragms may fail. This is caused by wet steam or excessive velocity due to undersizing
For more accurate control of downstream pressure, a pressure sensing pipe should be used
Adequate drain point should be fitted upstreamof the valve to control valve seat wearand erosion due to wet steam
Trainer [email protected] DP143: Salient Features
Valve can be used in superheated conditions. Stainless steel internals resist corrosion and erosion
Diaphragm operation gives high reliability & life expectancy and reduces the possibility of sticking due to dirty conditions
Wide range of control with four colour coded springs that give very accurate control of downstream pressure
12:1 pressure reduction ratio Easy adjustment. Springs can be changed without
turning off steam on applications where frequent changes of pressure are necessary
Excellent no flow characteristics so that there is no pressure creep on periods of no demand
Trainer [email protected] BRV: Salient Features
Long life phosphor bronze bellows and stainless steel internal parts
Robust & Simple In built strainer provides added protection Reduced vibration and noise on water applications
thanks to balanced , well damped valve design Choice of three easily interchangeable colour
coded pressure control springs Option of external downstream pressure sensing
for increased control sensitivity Security of set pressure by use of tamper proof pin
inside hand wheel Quick in-line maintenance through use of modular
internals reduces down time and maintenance costs
No multiple joints to leak - only one recessed body gasket
Trainer [email protected] BRVs
Principle of Operation
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.
Trainer [email protected]
Self Acting Control with 2 Port Valve
Actuator to Valve Connection
Adjustment Knob
Sensor
Add 1ºC to Sensor
Overload Bellows
Capillary
Movement caused by Adding Temp to Sensor
Thrust Pin
Valve Plug Movement
Valve Housing
Trainer [email protected] Effect of Raising the Set Value on
Self Acting Controls
78.5ºC 100% Load Value
80ºC Desired Value
81.5ºC Set Value
Set Value ºC
+1.5ºC
-1.5ºC
P Band ºC
P-band 0 to 100% Load
Set Value moved up 1.5ºC to 81.5ºC
3ºC
Trainer [email protected]
Installation Advice
Sensor requires adequate room for installation
Full immersion in good flow conditions
Pockets for fluid systems
Correct valve sizing
By-pass for heating systems with secondary mixing valves
No screwed valves on thermal oil systems
Fixed bleed should be offered on normally closed valves
Keep capillary lengths as short as possible
Keep pipework adequate supported for heavy products
Trainer [email protected] Cost Of Not Having A TR 121
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
Trainer [email protected] Safety Valves: Salient Features
Cast Steel Safety Valve
Clean bore, top guiding Pressure tightness upto blow off pressure
coupled with pressure tightness on reseating The use of ball pivot point so that the valve
disc can accurately align itself with the seat irrespective of the temperature distortion of surrounding components
Protection of the spring from the main flow of steam when discharging, making sure that it is not affected by the temperature of the steam
An adjustable blow-down ring is provided in order to obtain good reseating performance
Trainer [email protected] Payback Calculation for PRS
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
Payback Period = 8 1/2 months
