Transcript Pressure and Temp Controls

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
Trainer A.R.KANADE
[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 A.R.KANADE
[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 A.R.KANADE
[email protected]
Where To Use?
BRV (Direct Bellows Action)
DRV (Direct Diaphragm Action)
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Small loads
On/Off application
Low maintenance
Compact design
Low cost
DP (Pilot Operated)
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Small to medium loads
High control accuracy
Wide product range variations
Ideal close to process control
Medium to large loads
Simple operation
Robust design
Mains pressure reduction
High pressure turndown application
Trainer A.R.KANADE
[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 A.R.KANADE
[email protected]
Amount of Droop
If valve is set on no load:
 DP17 / DP143
 BRV
1/2”
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3/4”
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1”
0.2 bar
20% of no load pressure
25% of no load pressure
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 decrease
If load decreases the control pressure will increase
Trainer A.R.KANADE
[email protected]
Features of Spirax PRVs
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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 A.R.KANADE
[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 upstream
of the valve to control valve seat wear
and erosion due to wet steam
Trainer A.R.KANADE
[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 A.R.KANADE
[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 A.R.KANADE
[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 A.R.KANADE
[email protected]
Self Acting Control with 2 Port Valve
Valve
Housing
Adjustment Knob
Valve Plug Movement
Actuator to Valve
Connection
Sensor
Movement caused
by Adding Temp
to Sensor
Add 1ºC
to Sensor
Thrust Pin
Overload
Bellows
Capillary
Trainer A.R.KANADE
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Effect of Raising the Set Value on
Self Acting Controls
Set Value ºC
Set Value moved up
1.5ºC to 81.5ºC
81.5ºC Set Value
+1.5ºC
80ºC Desired Value
-1.5ºC
78.5ºC 100% Load Value
P Band ºC
P-band 0 to 100% Load
3ºC
Trainer A.R.KANADE
[email protected]
A Typical Self Acting Control
Trainer A.R.KANADE
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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 A.R.KANADE
[email protected]
Cost Of Not Having A TR 121
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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
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Recommended:1/2” TR 121
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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
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This extra consumption could occur every 10 minutes. By
the use of a TR 121, this can be eliminated
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SAVINGS
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4.5 Kg steam / 10 minutes
27 Kg/hr
130 Tons/Yr (4800 working hours)
Rs. 19,500 yearly
3 MONTH PAYBACK PERIOD
Trainer A.R.KANADE
[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 A.R.KANADE
[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/hr
Latent 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/hr
In 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. 70,000
A similar PRS would cost Rs. 50,000
Payback Period = 8 1/2 months
Trainer A.R.KANADE
[email protected]
Pressure Reducing Station