Closed discharge

This condition cannot be corrected on pumps with an enclosed impeller. You need to relax the restricted discharge flow on the pump. The problem could be a clogged downstream filter, a closed discharge valve, an over-pressurized header (back-pressurizing the pump), or a... 

No discharge pressure. Pump improperly primed. Inadequate Speed. Not enough NPSHa. Plugged impeller or piping. Incorrect rotation. Closed discharge valve Air aspirated or air pockets at the suction line. 

Each thermal expansion PR valve should be provided with an inlet CSO valve (and an outlet CSO valve in the case of closed discharge) to permit isolation for inspection and testing, if permitted by local codes. 

Fatigue Failure - The combined discharge header system should be designed in accordance with the same considerations of potential piping fatigue failure as described below for closed discharge systems. 

Turbine pumps are positive-displacement pumps They cannot be started with a closed discharge valve and require the fitting of a pressure relief valve to provide overpressure protection. 

The minimum required NPSH on the pump curves is normally determined using water at 60°F with the discharge line fully open. However, even though a pump will run with a closed discharge line with no bypass, there will be much more recirculation within the pump if this occurs, which increases local turbulence and local velocities as well as dissipative heating, both of which increase the minimum required NPSH. This is especially true with high efficiency pumps, which have close clearances between the impeller and pump casing. 

The fluid temperature increase was calculated using the equation for barrel rotation using Eq. 7.93. Like the equation for screw rotation, this equation was developed for a screw pump with a closed discharge. 

The driver s starting torque capabilities shall exceed the speed-torque requirements of the driven equipment. Unless otherwise specified, the motor shall be capable of accelerating the pump to rated speed at 80 % voltage against a closed discharge valve. 

There are many types of closed discharge systems and they can be very complex. They must be subject to careful, individual analysis of piping. 

If we combine the flow generated by the down channel and cross channel flows, a net flow is generated in axial or machine direction (w ) of the extruder, schematically depicted in Fig. 6.5. As can be seen, at open discharge, the maximum axial flow is generated, whereas at closed discharge, the axial flow is zero. From the velocity profiles presented in Fig. 6.5 we can easily deduce, which path a particle flowing with the polymer melt will take. 

The screw extruder is equipped with a die, and the flow rate of the extruder as well as the pressure rise at a given screw speed are dependent on both, as shown in Fig. 6.16. The screw characteristic line at a given screw speed is a straight line (for isothermal Newtonian fluids). This line crosses the abscissa at open discharge (drag flow rate) value and the ordinate at closed discharge condition. The die characteristic is linearly proportional to the pressure drop across the die. The operating point, that is, the flow rate and pressure value at which the system will operate, is the cross-point between the two characteristic lines, when the pressure rise over the screw equals the pressure drop over the die. 

The pressure profile thus depends on a number of variables geometrical (Ho, Hi, and L), operational (q, Vo), and physical property variables, and the viscosity fi. The maximum pressure that can be attained is = 1 (z = L) at closed discharge condition (q = 0)... 

In the normal stress extruder we first want to evaluate the pressure at the center as a function of disk outer radius, frequency of rotation, and rheological properties. We do this in the absence of radial flow (i.e., for a closed discharge condition), which will give us the... 

Example 6.9 The Maximum Pressure in the Normal-Stress Extruder Calculate the maximum pressure (at closed discharge) in a normal stress extruder of two 25-cm radius disks at 0.5 cm apart, shearing Low Density Polyethylene (LDPE) at 60rpm and 200°C. 

Equation 9.2-5 can be represented by plotting the flow rate Qs versus the pressure rise APs. Such plots, called screw characteristics, appear in Fig. 9.4. The intersection with the ordinate gives the drag-flow rate value and that with the abscissa, the maximum pressure at closed discharge. For isothermal flow of a Newtonian fluid in the absence of leakage flow,... 

The total power input is at minimum under open discharge conditions (Q = Qd) and is at maximum under closed discharge conditions (Q = 0). However, in addition to the power consumption in the channel, we must account for the power consumption in the flights,5 which is by no means negligible (la). 

Thus, the operating conditions affect the down-channel velocity profile, but not the crosschannel velocity profile. At closed discharge conditions (Qp/Qd = — 1), both the down-channel and cross-channel velocities vanish at / 2/3, implying that the whole plane at... 

Fig. 9.15 Mean strain as a function QpjQd ratio with helix angle as a parameter for IIH = 1. When QpjQd = — l, y goes to infinity this condition corresponds to a closed discharge operation.

We recall that at closed discharge conditions fluid particles stay at a fixed axial position. This means that the fluid in the crosshatched incremental volume Ay = nNDHAl does not leave it. Some fluid is dragged over the flight and recycled to the bulk, and since within the volume element the fluid circulates due to the drag of the barrel surface, we can model the system as a well-stirred tank with recycle, as shown in Fig. 9.17. 

Downstream from the mixing element section there are two separate inverse SSEs (see Section 6.2) that have the helical channel machined into the barrel, thus needing only rotating shafts to convey the material. Two pressure transducers, one diameter apart, record the pressure built up at closed discharge, a parameter that can be used to measure the viscosity of the molten blend at various shear rates. This rheometry section is the forerunner of the Helical Barrel Rheometer (HBR) of the Polymer Processing Institute (118). 

A centrifugal pump handles 220°F (377 K) water and has a shutoff head (with closed discharge valve) of 3200 ft (975 m). At shutoff, the pump efficiency is 17 percent and the input brake horsepower is 210. What is the minimum safe flow through this pump to prevent overheating at shutoff Determine... 

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