Pressure backpressure

It is important to minimize the rotation speed of the screw. High speeds can be used, in conjunction with the appropriate backpressure, to mold thin and/or long parts. Injection pressure should also be as low as possible. Dimensional stability improves when pressure is lowered because of a reduction in the residual stresses in the molded part. Injection pressure may have to be increased if an improvement in the weld line or a reduction in sink marks is desired. Part design and equipment capability should always be considered in the selection of injection pressure. Backpressure should be kept at the lowest possible value. Increasing it can sometimes help increase the stock temperature. 

At a holdup time longer than 10—15 min at a high temperature, resin degradation is avoided by keeping the rear of the cylinder at a lower temperature than the front. At short holdup times (4—5 min), cylinder temperatures are the same in rear and front. If melt fracture occurs, the injection rate is reduced pressures are in the range of 20.6—55.1 MPa (3000—8000 psi). Low backpressure and screw rotation rates should be used. 

Continuous Pressure Filters These filters consist of conventional drum or disk filters totally enclosed in pressure vessels. Filtration takes place with the vessel pressurized up to 6 bar and the filtrate discharging either at atmospheric pressure or into a receiver maintained at a suitable backpressure. Cake discharge is facilitated through a dual valve and lock-hopper arrangement in order to maintain vessel pressure. Alternatively, the discharged filter cake can be reslurried within the filter or in an adjoining pressure vessel and removed through a control valve. 

Safety Relief Valves Conventional safety relier valves (Fig. 26-14) are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting or the valve is reduced by the amount of superimposed backpressure expecied. Higher built-up backpressures can result in a complete loss of continuous valve capacity. The designer must examine the effects of other relieving devices connected to a common header on the performance of each valve. Some mechanical considerations of conventional relief valves are presented in the ASME code however, the manufacturer should be consulted for specific details. 

Balanced safety relief valves may be used in systems where built-up and/or superimposed backpressure is high or variable. In general, the capacity of a b anced valve is not signincantly affected by backpressures below 30 percent of set pressure. Most manufacturers recommend keeping tne backpressure on balanced valves below 45 to 50 percent of the set pressure. 

Provide adequate flow area (cross section for pressure relief without imposing high backpressure)... 

HEM for Two-Phase Pipe Discharge With a pipe present, the backpressure experienced by the orifice is no longer qg, but rather an intermediate pressure ratio qi. Thus qi replaces T o iri ihe orifice solution for mass flux G. ri Eq. (26-95). Correspondingly, the momentum balance is integrated between qi and T o lo give the pipe flow solution for G,p. The solutions for orifice and pipe now must be solved simultaneously to make G. ri = G,p and to find qi and T o- This can be done explicitly for the simple case of incompressible single-phase (hquid) inclined or horizontal pipe flow The solution is implicit for compressible regimes. 

The steam turbines in most of the large power plants are at a minimum divided into two major sections the High Pressure Section (HP) and the Low Pressure Section (LP). In some plants, the HP section is further divided into a High Pressure Section and an Intermediate Pressure Section (IP). The HRSG is also divided into sections corresponding with the steam turbine. The LP steam turbine s performance is further dictated by the condenser backpressure, which is a function of the cooling and the fouling. 

The gas, condensate, and free water are then discharged from the ncs scl through backpressure and liquid dump valves. The gas leaving the separator is saturated with water vapor at the temperature and pressure of the top of the low temperature separator. If this temperature is low enough, the gas may be sufficiently dehydrated to meet sales specifications. Dehydration is discussed in greater detail in Chapter 8. 

The set points for pilot-operated and balanced-bellows relief valves are unaffected by back-pressure, so they are able to tolerate higher backpressure than conventional valves. For pilot-operated and balanced-bellows relief valves, the capacity is reduced as the back-pressure goes above a certain limit. 

The backpressure is represented by the straight lines labeled minimum, normal and maximum. Only one capacity curve is shown since the increase in capacity resuldng from the lower steam pressure is negligible [4]. 

This valve provides an internal design (usually bellows) above/on the seating disk in the huddling chamber that minimizes the effect of backpressure on the performance of the valve (opening pressure, closing pressure and relieving capacity) [35]. See figures 7-4, 7-6, and 7-6A. 

Figure 7-6. Effect of backpressure on set pressure of safety or safety Construction of Pressure-Relieving Systems in ReHneries, API RP-520,...

This type of valve may be used when the variations in backpressure on the valve discharge connection do not exceed 10% of the valve set pressure, and provided this backpressure variation does not adversely affect the set pressure. 

Thus, if the dowmstream or backpressure on tlie valve is less than 53%-60% (should be calculated) of the values of P, note above, critical (sonic) flow wll usually exist. If the downstream pressure is over approximately 50% of the relief pressure, P, the actual critical pressure should be calculated to determine the proper condition. Calculation of critical pressure [29] ... 

Calculations of Orifice Flow Area for Conventional Pressure Relieving Valves, and Flow is Critical (sonic) Through Part of Relieving System, i.e., backpressure is less than 55% of the absolute relieving pressure (including set pressure plus accumulation). See Figure 7-7A, use... 

Kv, = 1.0 (Figure 7-26), constant backpressure with variation not to exceed 10% of set pressure. 

AP = P] — p2 = upstream pressure, psig (set -I- overpressure) — total backpressure, psig... 

Calculations of Orifice Flow Area using Pressure Relieving Balanced Bellows Valves, with Variable or Constant Back Pressure. Must be used when backpressure variation exceeds 10% of the set pressure of the valve. Flow may be critical or non-critical for balanced valves. All orifice areas. A, in sq in. [68]. The sizing procedure is the same as for conventional valves listed above (Equations 7-10 ff), but uses equations given below incorporating the correction factors K, and K,, . With variable backpressure, use maximum value for P9 [33a, 68]. 

If the ratio of backpressure to inlet pressure to valve exceeds the critical pressure ratio, Pc/Pj,... 

F2 = coefficient of subcrilical flow, see Figure 7-29 T = relieving temperature of inlet gas or vapor, °R P = upstream relieving pressure, psia, = set pressure + allowable overpressure + atmospheric pressure, usually 14.7 psia), psia P2 = backpressure on valve, psia W = required flow through valve, Ibs/hr V = vapor flow required through valve, standard cu ft/min at 14.7 psia and 60°F... 

P2 = backpressure or exit pressure, psia Pi = upsu eam reliewng oresstire, psia... 

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