1 Selection principle
According to the system requirements of steam condensate and the characteristic parameters of the steam trap, the principles of steam trap selection are expounded from five aspects of supercooling degree, back pressure rate, pressure difference, drain volume and steam pressure.
Subcooling is both a system requirement and a characteristic parameter of a trap.
(1) When the steam equipment requires fast heating speed, strict temperature control, no accumulation of condensate, and strict guarantee of heat transfer power, the system requires the minimum degree of supercondensation of steam condensate, and the priority should be the minimum (≤5℃) Mechanical trap.
(2) When the steam equipment does not have strict requirements for heating speed and temperature, but the accumulated condensate will interfere with the stable operation of the steam equipment, although the system does not have a clear demand for the supercooling degree of steam condensate, it should be preferred in principle. Mechanical trap with minimum coldness (≤5°C).
(3) When the steam equipment needs the use of condensate sensible heat (heat tracing piping, etc.), the system requires a large degree of supercondensation of steam condensate, and the heat with a large degree of supercooling and adjustable (≥15℃) should be used Static trap.
1.2 Back pressure rate
The back pressure rate is a system requirement, and the allowable back pressure rate is a characteristic parameter of the trap.
PX = (P2/ P1) ×100%( 1)
Py = (P4/ P3) ×100% (2)
In the formula, PX is the back pressure rate; Py is the allowable back pressure rate; P1 is the operating pressure of the trap inlet, MPa; P2 is the operating pressure of the trap outlet, MPa; P3 is the experimental pressure of the trap inlet, MPa; P4 is the trap Maximum experimental pressure at the outlet, MPa.
(1) This formula assumes that P1> P2, P3> P4.
(2) The formulas for the back pressure rate and the allowable back pressure rate are the same, but the meanings are different. The back pressure rate refers to the percentage of the operating pressure before and after the trap under the actual operating environment, which is the system requirement; the allowable back pressure rate is the maximum valve that gradually increases the pressure after the trap under different steam pressure conditions until the trap cannot discharge condensate normally The average value of the rear pressure and the percentage of the pressure before the valve is the characteristic parameter of the trap. Due to the stable environment and medium operation in the experiment, the back pressure changes slowly, so the allowable back pressure rate is actually the ambition or maximum value of the trap. In actual use, it is necessary to consider the changes in conditions to the trap action. The impact (such as environmental changes, load changes, etc.), so it is necessary for the trap to allow the back pressure rate to be greater than the actual back pressure rate of the steam condensate system. The difference between the two should not be too small, or even flat.
(3) No backpressure steam condensate system requires no trap type.
(4) For the steam condensate system with back pressure, the mechanical trap with the highest allowable back pressure rate (80%) should be preferentially selected; if it is restricted by conditions such as device space, device method, supercooling degree, etc., the allowable back pressure rate can be used instead Moderate (50%) thermodynamic steam trap; when the steam condensate system needs a large degree of subcooling (heat tracing piping, etc.), select the thermostatic trap with the lowest allowable back pressure rate (30%) . For example: when P≥0.10MPa in general petrochemical plant, and the condensate with discharge volume ≥100kg/h is recovered through the pipe network, the mechanical trap with the highest back pressure rate is preferred; otherwise, P<0.10MPa, and The condensate with a discharge volume of <100kg/h has no recovery value due to low pressure, large resistance along the way, and low thermal energy. When it can be discharged into the trench on the spot, select a thermostatic trap with the minimum back pressure rate.
1.3 Differential pressure
The pressure difference is the system requirement, and the minimum allowable pressure difference is the characteristic parameter of the trap.
ΔP1= P1－ P2( 3)
ΔP2= P4－ P3( 4)
(1) This formula assumes that P1> P2, P3> P4.
(2) Differential pressure ΔP1 and minimum allowable differential pressure ΔP2 are different. Pressure difference refers to the operating pressure difference between the front and back of the trap under actual operating environment, which is the system requirement; the minimum allowable pressure difference refers to the maximum valve that gradually increases the pressure after the trap under different steam pressure conditions until the trap cannot discharge condensate normally The average value of the back pressure and the pressure before the valve is the characteristic parameter of the trap. Due to the stable environment and medium operation in the experiment, the pressure changes slowly after the valve, so the minimum allowable pressure difference is actually the value of the trap or the minimum value that can be reached. In actual use, it is necessary to consider the conditions to change the action of the trap. The impact (such as: environmental changes, load changes, etc.), so the actual pressure difference of the steam condensate system is greater than the minimum allowable pressure difference of the trap. When the two are flat, the trap will not work properly.
(3) When the steam user is the equipment rather than the pipeline, the pressure of the steam pipe network is not equal to the operating pressure of the steam trap inlet. Because of the sudden increase in volume (steam enters the equipment from the pipe network), the operating pressure of the steam trap inlet must be less than the steam The pressure of the pipe network is generally taken as the operating pressure at the inlet of the steam trap = the pressure of the steam pipe network-0.05 to 0.1 MPa (G). The operating pressure at the outlet of the steam trap is taken to be the highest operating back pressure after the valve (0 when discharged directly to the atmosphere). The minimum pressure difference under this working condition and the calculated drainage amount look-up table can determine the type of steam trap. The pressure difference is proportional to the amount of drainage. When the trap is operating, the pressure difference increases. The amount of drainage can increase accordingly. This can ensure that even if there is a large change in pressure behind the valve, the trap can work normally.
Drainage is not only a system requirement, but also a characteristic parameter of a trap.
1.4. 1 Condensate in the pipeline during start-up
In the formula, G1 is the amount of condensate in the pipeline at the start of operation, kg/h; W1 is the weight of the pipeline, kg; W2 is the weight of the insulation material, kg; CP1 is the specific heat capacity of the steel pipe, kJ/( kg·℃), (carbon steel is 0. 469, alloy steel is 0.486); CP2 is the specific heat capacity of thermal insulation material, kJ/( kg·℃), (usually taken as 0.837); TS is the steam temperature, ℃; T0 is the atmospheric temperature, ℃; H1 is the operating condition Lower full steam enthalpy, kJ / kg; H2 is the full condensate enthalpy under operating conditions, kJ/kg; L is the pipe length, m.
1. 4.2 Heat dissipation of the pipeline during production
Α = 1. 163 × (6 + ω × 1 /2)
In the formula, G2 is the heat dissipation of the pipeline during production, W/( m·h); TS is the steam temperature, ℃; T0 is the atmospheric temperature, ℃; λ is the thermal conductivity of the insulation material, W/( m ・℃), (Usually rock wool is 0.043); D1 is the inner diameter of the insulation layer, m; D0 is the outer diameter of the insulation layer, m; α is the heat dissipation coefficient of the surface of the insulation layer facing the atmosphere, W / (m2·℃); ω is the wind speed, m/s.
1. 5 Steam pressure
Steam pressure is the system requirement and is used to determine the maximum operating pressure of the trap. The steam pressure here refers to the pressure of the steam system, which is the system requirement; the maximum allowable pressure refers to the maximum steam pressure that the trap can accept and is the characteristic parameter of the trap. Usually the maximum allowable pressure of the steam trap is> the pressure of the steam system, but the difference cannot be too large. Due to the high allowable pressure (sometimes referred to as the nominal pressure) of the trap, the amount of trap will be reduced correspondingly under low operating pressure, which directly affects the condensation Water drainage effect.
2 Analysis of common faults in the use of traps
2.1 Trap leakage
Traps usually need to be hot after the equipment is put into use, but after a period of operation, there will always be leakages in some traps, such as wire plugs, large cover gaskets, flanges and so on. This kind of leakage is mainly caused by the quality of the equipment or the unevenness of the flange surface, and the usual thermal shock is too large or the surface of the water hammer is formed. Such problems should be related to the quality of the equipment and the steam trap. Tighten the fluorine tape or replace the gasket. If the gasket is leaking and not replaced in time, it will damage the sealing surface of the trap and the valve body.
2.2 Trap blocking
The trap is fouled, not blocked by foreign objects or the capacity of the drain is reduced, and it can usually be discriminated from the operating parameters. Generally, the lever float ball trap itself does not have a filter screen, and a pipeline filter should be installed in front of the trap and cleaned regularly. The equipment should be kept level, and the water seal should be ensured when using.
2.3 The anti-freezing problem of traps in the north
The winter in the north is relatively cold. In the petrochemical equipment that is driven in the winter, many traps are easily frozen and cannot work properly. In cold areas, the thermodynamic type steam trap should be selected, but the mechanical type is not suitable. If it is used, anti-freezing measures should be taken. When not in use, the water in the equipment and the trap should be drained in time. Under the normal working condition of the steam and condensate recovery pipeline network, the trap will not freeze. If the trap freezes, it may be caused by the long-term shutdown of the steam pipe network. Frost damage usually occurs in mechanical traps because there is water in the valve body. In the location where the trap may freeze, the selection of the trap should be considered from the following aspects: (1) Material. Can not choose the steam trap of cast iron shell, need to use cast steel material. (2) Install an antifreeze valve on the trap. When the temperature of the condensed water in the valve body drops, the water is automatically drained to avoid the trap from freezing.
Steam traps are considered as small accessories in all steam systems, but they have a great impact on system operation and economic operation. Therefore, in addition to the selection of steam traps and pipeline design, the protection and maintenance of steam traps and the analysis and elimination of common faults are also crucial. Only by fully paying attention to the important role of the steam trap in production and operation, and performing diligent protection and regular maintenance to keep the trap in good working condition, can we ensure the best energy-saving effect and improve economic efficiency.