What is a steam valve

What is a steam valve?

The steam valve is to control the opening of the opening and closing parts in the valve body to adjust the flow of the medium and reduce the pressure of the medium. At the same time, it can adjust the opening of the opening and closing parts with the help of the pressure behind the valve to keep the pressure behind the valve within a certain range. Under the condition of the constant change of the inlet pressure, it can keep the outlet pressure within the set range and protect the subsequent life and production appliances.

The steam valve is generally installed horizontally and controlled by the pressure behind the valve. When the pressure sensor detects that the valve pressure indication increases, the opening of the steam valve decreases; when the steam valve is detected, the opening of the steam valve increases to meet the control requirements.
Steam valve – the pressure reducing ratio of the valve must be higher than the system value to a certain extent; even at the maximum or minimum flow, it should be able to respond to the positive or negative action control signal. These valves should be selected for a useful control range of 20% to 80% of the maximum flow. It is normal to be equal proportion type or has equal proportion characteristic. These types of valves have the best flow characteristics and flow range required by proportional control.

Characteristics of steam valve

  • 1. High control precision, stable, safe and reliable.
  • 2. The temperature sensing liquid is compressed evenly and controlled by proportional regulation.
  • 3. The temperature setting is easy to operate and adjust.
  • 4. Small size, light weight, easy installation.

Classification of steam valve

According to the structure, it can be divided into diaphragm type, spring diaphragm type, piston type, lever type and bellows type; according to the number of valve seats, it can be divided into single seat type and double seat type; according to the position of valve disc, it can be divided into positive action type and reaction type.

There are many kinds of steam valves, including pilot piston steam valve, diaphragm steam valve, bellows steam valve, proportional steam valve, self operated steam valve, direct acting piston steam valve, back pressure regulating valve, etc. They are suitable for different working media.

In the steam valve, the mechanism that automatically regulates the downstream pressure usually makes use of the force balance between the steam pressure and the regulating spring. At present, this is almost all steam valve used by the general concept. However, there are two different ways to achieve this mechanism to control the valve opening:
Non pilot, direct acting steam valve: the adjusting spring directly applies the setting force of the downward pressure to the main valve.
Pilot type steam valve: the adjusting spring directly applies the setting force of downward pressure to the pilot valve, which is smaller and different from the main valve.

Direct acting steam valve (without pilot)

For small loads that do not require very precise pressure control.
Advantages: small size, low price, easy to install.
Disadvantages: higher pressure drop (vs. set pressure change) than pilot PRV.
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The output steam pressure is regulated by the balance force acting directly on the valve itself: the downward force generated by the compression regulating spring resists the upward force from the secondary pressure acting on the bottom of the bellows or diaphragm.

In the direct acting steam valve, the opening of the valve directly depends on the movement of the adjusting spring. If the spring is compressed, it creates an opening force on the valve, which increases the flow.
When the pressure accumulates downstream, the downstream pressure is transferred to the bottom surface of the adjusting spring (usually close to the bellows or diaphragm), where the upward force is balanced with the compression force of the spring. The spring pressure to open the valve is limited to achieve sufficient spring sensitivity while offsetting downstream pressure changes. The end result is a simple control of the pressure at the high flow rate that causes the pressure drop through the valve orifice.

Pilot-operated staem valves 

For larger loads where very precise pressure control is necessary.
Advantages: compared with the direct acting type, it can control the pressure accurately and respond quickly to the load change, and can be used in a larger flow range.
Disadvantages: large size, high price.
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The output steam pressure is regulated in a way similar to the direct acting PRV, but indirectly through the pilot valve.
When the pilot valve is activated, it provides much more open flow to the larger main valve than the pilot valve itself. Then, the pilot valve is balanced by the secondary pressure to control the opening flow to the main valve accordingly.
In the pilot steam valve, the pilot valve is used to load the piston or diaphragm, thus increasing the downward force used to open the larger main valve. In this way, the flow rate can be increased with lower pressure compensation (depressurization). The opening and closing of the pilot valve is controlled by the force balance between the regulating spring and the secondary pressure, and the action mode is the same as that of the direct acting valve.
But in the pilot PRV, the opening and closing of the pilot valve will purposefully transfer pressure to the main valve piston or diaphragm. Then, the pilot flow pressure will produce a downward force, which will be amplified by the piston or diaphragm surface, so that the main valve will open more and the flow will reach a very high level.
Because the use of piston or diaphragm will amplify the downward force, the slight change of pilot valve opening will lead to a large change in the flow through the main valve and downstream pressure. Therefore, in order to achieve rapid response in a large steam flow range, the spring force on the regulating valve almost does not need to be changed. Compared with direct acting valve, fast response and tight pressure control are the main advantages of this valve.

Application of steam valve

Pressure reducing valve is a kind of intelligent valve which uses the energy of medium itself to regulate and control the pipeline pressure. It is used for domestic water supply, fire water supply and other industrial water supply systems. The outlet pressure of the main valve can be adjusted through the pressure reducing pilot valve of the regulating valve. The outlet pressure does not change with the change of inlet pressure and inlet flow. The outlet pressure can be maintained at the set value safely and reliably, and the set value can be adjusted according to the need to achieve the purpose of decompression. The valve has the advantages of accurate pressure reduction, stable performance, safety and reliability, convenient installation and adjustment, and long service life.
The steam valve mainly controls the fixed outlet pressure of the main valve. The outlet pressure of the main valve does not change due to the change of the inlet pressure or the change of the outlet flow of the main valve. It has the function of improving the system operation condition and potential water saving. According to statistics, its water saving effect is about 30%. It is suitable for industrial water supply, fire water supply and domestic water pipe network system. It is widely used in high-rise buildings, areas where the water pressure of urban water supply network is too high, mines and other occasions to ensure that each water point in the water supply system can obtain the appropriate service water pressure and flow.

Working principle of steam valve

The steam valve reduces the inlet pressure to a certain required outlet pressure by throttling the opening and closing parts, and keeps the outlet pressure stable. When the steam valve leaves the factory, the adjusting spring is in the uncompressed state, and the main valve disc and the auxiliary valve disc are in the closed state. When in use, turn the adjusting screw clockwise to compress the adjusting spring, so that the diaphragm disc moves to open the auxiliary valve disc, and the medium enters the piston from hole a through the auxiliary valve seat to hole B. under the action of medium pressure, the piston moves downward to push the main valve disc away from the main valve seat, so that the medium flows To the rear of the valve. At the same time, it enters into the lower part of the diaphragm through hole C. when the pressure behind the valve exceeds the set pressure, it pushes the diaphragm upward to compress the regulating spring, and the secondary valve disc moves to the closing direction, so that the medium flowing into the piston decreases and the pressure also decreases. At this time, the main valve disc moves up and down under the push of the spring force of the main valve disc, so that the gap between the main valve disc and the main valve seat decreases, and the medium flow also decreases On the contrary, when the pressure behind the valve is lower than the set pressure, the clearance between the main valve disc and the main valve seat increases, and the medium flow also increases, so that the pressure behind the valve increases to reach a new balance.

Use of steam valve

Installation of steam valve

The steam valve Y43H-16 is equipped with 0.05 ~ 0.4MPa, Y43H-25, 1 ~ 1.6Mpa, Y43H-40, 1 ~ 1.6Mpa, 1.6 ~ 2.5MPa adjusting spring, Y43H-64 and 1 ~ 3Mpa adjusting spring. The valve is equipped with 0.1 ~ 1MPa adjusting spring when leaving the factory, and the rest is attached with the valve. The user can choose according to the outlet pressure value.
The pipeline system must be washed and cleaned before installing the steam valve. In order to prevent welding slag, oxide skin, etc. from flowing into the valve, affecting the normal operation of the valve.
The valve should be installed in a place convenient for operation and maintenance, and it must be installed vertically on the horizontal pipeline. Attention should be paid to make the flow direction of medium in the pipeline consistent with the direction indicated by the arrow on the valve body, and do not reverse install.
When the steam valve is installed and used, the stop valve of the bypass pipeline should be opened to eliminate the mixture of condensate water and steam water in the pipeline, so as to prevent the steam valve from being damaged by garbage when the steam valve is opened. When there is no abnormal phenomenon, slowly rotate the adjusting screw clockwise to adjust the outlet pressure to the required pressure (based on the gauge pressure behind the valve). After adjustment, back tighten the lock nut, Screw on the protective cover.
A filter should be installed in front of the steam valve to prevent impurities in the medium from entering the steam valve and affecting its performance.
There should be a straight pipe before and after the steam valve. The length of the straight pipe in front of the valve is about 600mm, and the length of the straight pipe behind the valve is about 1000mm.
Generally, pressure difference between inlet and outlet of steam valve must be ≥ 0.2MPa.
The steam valve usually has various specifications such as dn50-dn 100, the working pressure before and after the valve is < 1MPa and 0.1-0.5mpa respectively, and the error of pressure regulating range is ± 5% – 10%.
It should be noted that although the water flow through the steam valve has a great head loss, it is still energy-saving in general because it reduces the waste of water, makes the flow distribution of the system reasonable and improves the system layout and working conditions.

Safety rate control

As an important secondary energy, steam is widely used in many industrial fields. The coal consumption of steam system accounts for 1 / 3 of the output, which has become an important area of energy consumption. The steam trap is an important equipment to ensure the normal operation of steam, and it is an important self-control valve for condensate recycling and energy saving. There are many kinds of traps, which can be divided into floating ball trap, thermal static trap, thermal dynamic trap, inverted bucket trap, etc.
Safety rate is the estimated safety factor that can ensure the normal operation of the steam valve when the actual condensate production of the steam using equipment is in error with the marked capacity when determining the capacity of the steam valve. This kind of safety rate is not only stipulated in theory and can not be obtained by calculation, nor is it completely derived from empirical data. Generally speaking, the best way to consider the safety rate is to ask the steam valve manufacturer directly. If in the selection of steam valve, the safety rate of its selection is improper, so that the capacity of steam valve is too large or insufficient, it will have very bad consequences.

The safety rate is too high

  • 1. The capacity of steam valve will increase the cost.
  • 2. If the steam valve is operated intermittently, excessive capacity will lengthen the operation cycle of the steam valve, increase the average retention of condensate, and reduce the capacity of steam equipment.
  • 3. For the steam trap with continuous (proportional) action like the floating ball trap, due to the small opening of the valve disc, excessive capacity will cause galling of the valve seat (when the high-speed fluid passes through the narrow gap, the corrosion of the contact surface will form a groove), which will damage the valve seat and cause leakage.
  • 4. Shorten the service life of the steam valve.

The safety rate is too low

  • 1. It can’t adapt to the load change of steam equipment, so that the operation efficiency is significantly reduced.
  • 2. The condensate through the steam valve often reaches the maximum limit, which makes the valve disc and seat prone to corrosive damage.
  • 3. Shorten the service life of the steam valve.

Matters needing attention

Hand wheel, handle and transmission mechanism are not allowed to be used for lifting, and collision is strictly prohibited.
The double gate valve should be installed vertically (i.e. the stem is in the vertical position and the hand wheel is at the top).
The gate valve with bypass valve should be opened before opening (to balance the pressure difference between the inlet and outlet and reduce the opening force).
The gate valve with transmission mechanism shall be installed according to the product manual.
If the valve is used frequently, lubricate it at least once a month.
For the convenience of operation and maintenance, the valve is generally installed vertically on the horizontal pipeline.
The steam valve must be installed in strict accordance with the direction of the arrow on the valve body to keep consistent with the direction of fluid flow. If the water quality is not clean and contains some impurities, a filter must be installed at the upstream water inlet of the steam valve (we recommend that the filtration accuracy is not less than 0.5mm).
In order to prevent overpressure behind the valve, a steam valve should be installed at least 4m away from the valve outlet.
The steam valve plays a certain check role in the pipeline. In order to prevent the harm of water hammer, a small expansion tank can also be installed to prevent damage to the pipeline and valve. The filter must be installed in front of the inlet pipe of the steam valve, and the expansion tank must be installed behind the outlet pipe of the steam valve.
If you need to install the steam valve in the hot water system, you must install a check valve between the steam valve and the expansion tank. In this way, the expansion tank can absorb the volume of water increased due to thermal expansion, and prevent the influence of hot water backflow or pressure fluctuation on the steam valve, so as to ensure the normal operation of the steam valve for a long time.

Why steam valve is needed in process control

Technically, a valve is a device used to control the flow of liquid or steam in any system. The main purpose of steam valve is to reduce the inlet steam pressure in the process. In addition to controlling the pressure, the steam valve also controls the temperature.
Boilers usually work at high pressure because low pressure operation causes water to be carried in steam. The low specific volume of high-pressure steam helps to reduce the weight of the pipeline. In fact, steam distribution becomes easier and cheaper due to the lower cost of piping and insulation materials.
Industrial processes use steam at lower pressures. The reason is that low-pressure steam has high latent heat, which greatly improves energy efficiency. The steam pressure is related to the temperature, so the temperature can be adjusted automatically by controlling the steam pressure. Reducing steam pressure is also related to the safety needs of the plant. Steam pressure can be controlled by steam valve.
In the equipment using steam, steam is usually generated under high pressure, and heat is provided for each steam user through local decompression. The purpose of the pressure reducing valve is to minimize the diameter of the steam transmission pipeline and make the steam transmission more cost-effective.

How to reduce steam pressure

A common way to reduce the pressure is to reduce the size of the steam passage by throttling. For the most basic decompression, the traditional stop valve can be simply installed in the fixed and partially open position, or the orifice plate can be inserted in the steam flow. However, any flow fluctuation will be accompanied by corresponding pressure fluctuation. To avoid this, a pressure reducing valve (PRV) can be used to precisely control the downstream pressure. These pressure reducing valves can automatically adjust the valve opening to ensure that the pressure remains unchanged even if the flow rate fluctuates.

Advantages of pressure reducing valve

By using the combination of start-up control valve, pressure sensor and controller, the pressure can be kept constant. However, the pressure reducing valve has the advantage of fully automatic independent operation to control the pressure without any type of external power. It can make extremely fast response action based on the immediate perception and adjustment of downstream pressure.
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Benefits from enabling the heating process to use steam below boiler pressure. Therefore, the pressure reducing valve is usually used to reduce the steam pressure to the set value required by the process.

Technical parameter

Parameter

Nominal pressure
(MPa)
Shell test pressure
(MPa)
Sealing test pressure
(MPa)
Maximum inlet pressure
(MPa)
Outlet pressure range
(MPa)
Pressure characteristic deviation
(MPa)△P2P
Flow characteristic deviation
(MPa)△P2G
Permeability
2.5
3.75
2.5
2.5
0.02-1.6
GB12244-1989
GB12244-1989
GB12245-1989

The shell test does not include diaphragm and valve cover.

Material of main parts

Part name
Material of parts
Valve body, valve cover and bottom cover
WCB
Valve seat and disc
2Cr13
Cylinder liner, piston
Aluminum iron cast copper
Diaphragm
1Cr18Ni9Ti
Guide valve seat, guide valve stem
2C13
Main valve spring
50CrVA
Main spring of pilot valve
50CrVA
Adjusting spring
60Si2Mn

Dimensions

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Structural drawing of steam pressure reducing valve

Nominal diameter (DN)
Dimensions
L
H
H1
1.6/2.5MPa
4.0MPa
15
160
180
265
85
20
160
180
265
85
25
180
200
280
95
32
200
220
280
95
40
220
240
290
105
50
250
270
300
130
65
280
300
315
145
80
310
330
330
170
100
350
380
405
200
125
400
450
450
225
150
450
500
485
265
200
500
550
570
290
250
650
640
325
300
800
740
360
350
850
810
395
400
900
950
430
500
950
1080
500
Nominal diameter (DN)
Dimensions
L
H
H1
6.4MPa
10.0/16.0MPa
15
180
180
280
100
20
180
200
280
100
25
200
220
300
115
32
220
230
300
115
40
240
240
315
130
50
270
300
340
135
65
300
340
355
150
80
330
360
370
175
100
380
450
205
125
450
510
235
150
500
555
275
200
550
640
300
250
650
720
335
300
800
830
375
350
850
880
410
400
900
1020
445
500
950
1080
515

Common fault treatment of steam valve:

The control valve does not work. The fault phenomena and causes are as follows:
1. No signal, no gas source.

  • ① The air source is not turned on;
  • ② the air duct is blocked or the filter and pressure reducing valve are blocked due to the water content in the air source freezes in winter;
  • ③ the compressor is faulty. 
  • ④ the air source main pipe leaks.

2. There is air supply and no signal.

Regulator failure, signal tube leakage, positioner bellows leakage, adjusting reticulum damage.
3. The positioner has no air source.

  • ① The filter is blocked;
  • ② the pressure reducing valve is faulty;
  • ③ the pipeline is leaking or blocked.

4. The positioner has air supply but no output. The orifice of the positioner is blocked.
5. Signal and no action.

  • ① The valve core falls off;
  • ② the valve core is stuck with the society or the valve seat;
  • ③ the valve stem is bent or broken;
  • ④ the valve core of the valve seat is frozen or burnt;
  • ⑤ the spring of the actuator is rusted due to long-term use.

The action of steam valve is unstable. The fault phenomena and causes are as follows:
1. The air source pressure is unstable.

  • ① The compressor capacity is too small;
  • ② the pressure reducing valve is faulty.

2. The signal pressure is unstable.

  • ① The time constant of the control system (t = RC) is not appropriate;
  • ② the output of the regulator is unstable.

3. The gas source pressure is stable and the signal pressure is stable, but the action of the control valve is still unstable.

  • ① The ball valve of the amplifier in the positioner is not closed tightly due to the wear of dirt, and the output vibration will occur when the air consumption is especially increased;
  • ② the baffle of the nozzle of the amplifier in the positioner is not parallel, and the baffle can not cover the nozzle;
  • ③ the output pipe and line leak air;
  • ④ the rigidity of the actuator is too small;
  • ⑤ the friction resistance of the valve stem is large, and there is a blocking phenomenon with the contact part.

Source: Network Arrangement – China Steam Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@ugsteelmill.com

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