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

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

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).

Where To Use? BRV (Direct Bellows Action) DP (Pilot Operated) 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

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

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: BRV Pressure Increase = Set Pressure / (1 - Droop %) If load increases the control pressure will decrease If load decreases the control pressure will increase

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

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

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

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

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.

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

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

A Typical Self Acting Control

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

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

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

Payback Period = 8 1/2 months 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

Pressure Reducing Station