The effect of the stability of condensed water to steam
1 Valve structure and the possibility of condensate water backup
We should say, pressure reducing valve is a more complex valves, structural is complex, internal valve sleeves valve (one valve has two distinct features pilot valve and main valve) must be in detail to understand their structures and relevant effect. For example, the most commonly used home-made y43h series piston reducing valve and imported pilot reducing valve for comparation and introduction. (See Figure 1 and 2)
1.1 The main structure and the related distinction
As can be seen from the diagram, either domestic or imported, though structure has some difference, but they are both composed by the adjustable spring component, pilot valve components, main valve components and adjustable channel four parts.
1.1.1 Regulating spring components and pilot components: In addition to the pilot valve spool shape difference, (one is cone units, one is sphere) others are almost the same.
1.1.2 Main valve components: the structure are completely opposite.
domestic main valve spool is arranged below the valve seat, the input gas direction is low in and high out, showing transverse s-shape;
Imported main valve spool is arranged above the seat, the input gas direction is high in and low out, showing anti-cross s-shape.
1.1.3 Pressure regulative channel (the α, β, γ, in Figure 1, a, b, c in Figure 2) arrangement is different
Domestic valve pressure regulative channels are preset in the internal (built-in), imported valve pressure regulative channel valve is connected to the outside body by copper pipes (external type).
1.1.4 β channel and b channel function distinction
Domestic β channel directly connect to the below piston cylinder upper cavity through pilot valve circular steam gas cavity, only with the connecting function.
Imported b channel connects to the downstream outlet body from the main gas supply channel branch (a very small hole), the role of this channel is very special, it not only form pressure differential with the main diaphragm below cavity, helping the main valve diaphragm movement, but also can drain away part of the residual pressure steam and condensate water to the downstream outlet pipe sections, more conducive to the rapid closure of the main valve.
China-made valve, as long as the valve is opened, when the condensate water will soon enter into the upper pilot valve annular steam cavity from α-channel, β-channel and the piston cylinder upper cavity, with the piston moves down, condensate water piled up until the piston stroke reachs the dead point, in this process, cylinder becomes a water tank. Domestic valve, due to structural features of water storage capacity is relatively small, so as long as a small amount of condensate water can fill these positions.
Imported valve, after the pilot valve opens, the condensate water is generally deposited on the cylinder, piston cylinder upper cavity and the main diaphragm lower cavity such main valve movement original actuator, and β, a, b channel such pressure transmission system. This shows that due to the characteristics of pressure reducing valve structure, valve condensate water backup is inevitable, and are the heart parts.
From the reducing valve working principles to analyze condensate water damage to the pressure stability and regulation
To further explore the effects of condensed water to reducing valve pressure stability and regulating function, first, we must understand the reducing valve working principle and mutual relations.
2.1China-made reducing valves waorking principles
Pilot valve opening are using the top regulative bolt clockwise twist, so that contraction produced by elastic springs, makes pilot valve diaphragm downward depression, acting on the valve rod and making it displace downward to open pilot valve. after the pilot valve opens, the upstream input steam pipe section a chamber steam through α-channel (gas supply regulates channel), through the pilot valve to enter into circular gas chamber, by the β-channel directly to the below piston cylinder upper chamber. In a continuous steam supply of the a chamber, the pressure continues to rise, push down piston to open the main valve, and steam flows steady from a cavity to b cavity. When the downstream outlet pipeline section b cavity load is satisfied, the more steam also causes b cavity pressure continuously increased. The rising pressure through γ channel (pressure sensing channels) to feedback to the pilot valve diaphragm down cavity, makes pilot valve diaphragm processed upward, overcomes the upper regulative spring pressure, pilot valve is closed small or closed. Thus, close small or closed the steam source from upstream α. channels. When the piston cylinder cavity pressure decreases, under the effect of below reset spring, the main valve is closed small or closed, then b cavity pressure began to drop, to achieve the purpose of regulating pressure with such a cycle.
2.2 imported reducing valve working principles
When the lead after the valve is opened, from Figure 2 we can see, the upstream pipe steam quickly enters into internal filter cover, to reach a channel through the pilot valve (steam supply regulating channels), when a channel is full of steam, it will be sent to the main valve diaphragm down chamber, while part of the steam through the b-channel (pressure control channel) has been diverted into the b cavity. The main valve diaphragm down chamber under a channel constant s steam upply, the main valve diaphragm processes upward after compression, the resulting thrust promotes the main stem moving upward, opening main valve, the same, steam flows steadily from a chamber to b cavity. When the downstream outlet pipeline section b cavity load is satisfied, the more steam also causes b cavity pressure continuously increased, the rising pressure through c channel (pressure sensing channels) to back to the pilot valve diaphragm down cavity, makes pilot valve diaphragm processed upward, overcomes the upper regulative spring pressure, pilot valve is closed small or closed, reducing and cutting off the a cavity steam. When the main valve diaphragm down chamber pressure gradually reduces, the upper part of the main valve spring affects, makes main valve spool down to back seat, the main valve has been closed small or off (the excess steam at the same time through the b-channel to release into the b cavity, the main valve has been rapid closed), makes the pressure of b cavity decreased, to achieve regulation. (the opening principles of valve, imported valves and valves made in China are the same, omitted)
2.3 Condensate water damage pressure stability regulating function mechanism analysis
When the downstream pressure rises (inner b cavity) and needs to downward pressure. Under normal circumstances b chamber ultra-high pressure, through pressure sensing channel (γ) to transmit to pilot valve, makes it closed small or off, thereby close small or cut off the steam source from a chamber. When the condensed water intrude into the cylinder, annular gas chamber, due to the characteristics of the incompressible water, at this time, main valve reset spring completely lost its role, the piston can not be upstream, main valve can not close, the entrance (a cavity) steam are still flooding in, through the normal open state main valve entering into the b cavity. To make ultra-high pressure and out of control.
When the system condensate water is full of a, b-channel and main diaphragm down cavity, under the thrust from a chamber steam, main valve diaphragm in up puff state. Similarly, characteristics of incompressible water, makes upper reset spring can not back seat downward, the main valve is also in the normal open state, stable regulatory function is damaged.
Here, we must stress that it is, though, reducing valve belongs to proportional regulating valve, however, when condensed water fills these core working area of inner valve, the proportional regulating nature is completely changed, the key issue lies in moving parts friction function and the time differential of the acting sequence, it determines main valve movement hysteresis nature. (pilot valve moves first, followed with main valve) when receiving the ultra-pressure message from the γ and c-channel, first of all, closing small or off pilot valve. In this way, cut off the escape route for the condensate valve, obviously, the piston and main valve diaphragm can not move.
Synthesizing the aboved analysis and discussion, it is clear that system condensated water has no benefit for heating supply project. From this conclusion, the system condensated water is the most basic reason of damaging steam reducing valve pressure stability and regulation functions.
3 The system condensate water is the only source
In the process of writing this article, peer-raised pressure reducing valve in its own working process heating whether will produce the condensated water? Raise this issue makes us consider whether internal reducing valve would precipitate condensate water problem? Therefore, it is necessary to understand the the thermal status in reducing valve working process. In order to visually illustrate the problem, analyzing lgp-i to see the pressure reduced circumstances, the all related state parameters change circumstance, there will be an answer.
Suppose the upstream inlet pressure is (before valve) p1, the downstream outlet pressure is p2, p1 and dry saturated steam line intersect at o1 along isenthalp down. Downstream outlet pressure p2 intersects isenthalp at o2, when the pressure decreased from p1 to p2, the other relevant parameters also take place corresponding changes.
As seen from Figure 3, the pressure fell to p2 from p1 along the isenthalp, so there is no change in enthalpy, but the entropy rises from s1 to s2, specific volume also increases from v1 to v2, but the temperature dropped from t1 to t2.
From this, the flow state of steam through the reducing valve, it should be considered to be adiabatic throttling process. This adiabatic throttling process is irreversible adiabatic expansion isoenthalpy flow process. After the steam throttling, the enthalpy value is unchangeed, specific volume and entropy increase, the temperature decreases slightly. After the steam through the valve, will not produce condensate water, and steam dryness has also been improved, can be said that the production of condensate water is from the system, and is unique. In this way, the measuring methods for the prevention and reduction of condensate water entering into the valve also become simple.
4 Measures and methods
4.1 a good practice of excluding condensate water in pipeline
Steam heat source to the steam end, along the way varies from tens of meters to several hundred meters or more, plus some equipments steam discontinuous, steam pipes continuous heat exchange with the environment, this part condensate water exclusion is commonly used sub-hydrophobic to solve. And what is the effect? From Figure 4, we can see that, because the sparse water pipe diameters we use are relatively small, it is conceivable that when the pipe steam flow in dozen to a few tens of meters/sec , while the actual situation is the condensated water is not drained a lot, it has been pushed through the sparse water pipes. this traditional practice with a low drainage efficiency, the effect is not ideal.
Between transmitting steam main pipes and sparse water pipes plus installed a section condensate collecting short tubes, the diameter is thick, catchment ability is large, sparse water pipes are leaded out from condensate water short tubes, to form water sealing and vapor is not easy to escape. The ratio of condensate water short-tube diameter and steam transmitting pipes is recommended as 1/2 ~ 2/3. If dn100 steam pipes, condensate water collecting short tubes diameter can choose 50 to 80 diameter. This approach with large displacement, the effect is good, now widely adopted abroad, it is worth learning.
4.2 Decompression device steamwater separator settings is essential to install steamwater separator, to the maximum extent to ensure the supply of dry steam, it is an important safeguard measure for decompression device stability work and using life. I design steamwater separator based on combination of gravity separation and blocking principles, the actual using effect is not bad, just for reference.
Looking abroad steamwater separator products, from the principle it is devided into gravity type, resistance condensate type, centrifugal type. From the form point, there are horizontal, vertical, species diversity, and complete specifications. But the domestic market is no product for selecting. Now it is mostly used products are foreign. (Expensive, prohibitively)
Condensate water retention is harmful to steam supply heat system. Therefore, the steamwater separator research and development have many things to do, hope colleagues to concern commonly.
4.3 some problems in decompression device design:
（1） the bottom of steamwater separator condensate water discharge port must be set a separate discharge port, for the drainage when the device is startup.
（2）The by-pass setting, preferably in the top of the reducing valve, or parallel setting.
（3） The steam filter is installed close to the reducing valve.
（4） The selection of valve flow should be right, and the amount of equipment steam consumption should be calculated accurately. In my experience, the reducing valve flow should be 10 to 20% larger than equipment steam consumption that is appropriate, avoid to appear "big horse-drawn small trolley case", otherwise it also has an impact for the stability of the pressure reducing conditions.
