Orifice limiting

The condition for Knudsen effusion that tire irreair free path of tire effusate should be less tlrair the diameter of the circular orifice, limits the usable vapour pressure of the material in the Knudsen cell to about 10 Pa, since the orifice diameter would have to be less than 1 mm at a higher pressure. 

The orifice-riser distributor is designed to lay the hquid carefully onto the bed, with a minimum of contact with gas during the process. It can be designed to provide a large number of liquid streams, with the limit of sufficient liquid head to provide uniform liquid flow through the orifices. The gas risers must oe designed to accommodate the expected variations in flow rate, often with a minimum of pressure drop. For veiy distribution-sensitive packings, it is necessaiy to include pour points in the vicinity of the column wall (to within 25 mm). 

Flow measurements using nonintrusive or low mechanical ac tion principles are desired, such as magnetic, vortex-shedding, or Coriolis-type flowmeters. Orifice plates are easy to use and reliable but have a limited range and may not be suitable for streams which are not totally clean. Rotameters with glass tubes should not be used. 

At design head, on the other hand, capacity does not change markedly with speed, so that once the design point has been passed the pump-turbine acts as a restriction in the hue. Since most or these units operate on a relatively fixed pressure differential, they then tend to act Eke an orifice to limit flow, and little or no benefit can be realized from any overcapacity in terms of fluid flow available to the unit in the actual iustaUatiou. 

ASMEflow nozzle. These nozzles provide for accurate measurements. Their use is limited because they are not easily placed in a process plant however, they are excellent for shop tests. Venturi meters and nozzles can handle about 60% more flow than orifice plates with varied pressure losses. 

Another type of interference in ICPMS is suppression of the formation of ions from trace constituents when a large amount of analyte is present. This effect depends on the mass of the analyte The heavier the mass the worse the suppression. This, in addition to orifice blockage from excessive dissolved solids, is usually the limiting factor in the analysis of dissolved materials. 

Blockages or faulty pumps. Pumps should be checked periodically as instructed by the manufacturer. They can be calibrated using rotameters or bubble flowmeters. Unless pumps possess a limiting orifice they should be calibrated with the air indicator tube in position. 

Restriction Orifice - In general a restriction orifice should not be used as a means of limiting the capacity of a pressurization path. In special cases, where large incentives apply (such as reducing die size of a flare system), a restriction orifice may be used, provided that all the following conditions are satisfied ... 

Flow Meter Orifice Plate - A flow meter orifice plate is permissible in normal process flow pressure reheving path, provided that it can pass the required emergency flow without exceeding pressure limits of the upstream equipment. However, it is not acceptable in PR valve inlets and flare headers. 

As normally designed, vapor flow through a typical high-lift safety reliefs valve is characterized by limiting sonic velocity and critical flow pressure conditions at the orifice (nozzle throat), and for a given orifice size and gas composition, mass flow is directly proportional to the absolute upstream pressure. 

We shall first consider the case of non-flashing liquids. In this situation, there is no critical flow pressure limiting the flow of liquid through a PR valve orifice, as opposed to the case of vapor flow. The discharge rate is a function of the pressure drop across the valve and can be estimated by the following expression ... 

Limit the total possible charge to a batch reactor by using a precharge or feed tank of limited capacity. Alternatively, limit the addition rate by selecting a pump with a maximum capacity lower than the safe maximum addition rate for the process, or by using restriction orifices. 

Use physical limits of pipe size, restrictive orifices, and pump sizing to limit excessive flow rates. 

The flow of a compressible fluid through an orifice is limited by critical flow. Critical flow is also referred to as choked flow, sonic flow, or Mach 1. It can occur at a restriction in a line such as a relief valve orifice or a choke, where piping goes from a small branch into a larger header, where pipe size increases, or at the vent tip. The maximum flow occurs at... 

In general, the sonic or critical velocity is attained for an outlet or downstream pressure equal to or less than one half the upstream or inlet absolute pressure condition of a system. The discharge through an orifice or nozzle is usually a limiting condition for the flow through the end of a pipe. The usual pressure drop equations do not hold at the sonic velocity, as in an orifice. Conditions or systems exhausting to atmosphere (or vacuum) from medium to high pressures should be examined for critical flow, otherwise the calculated pressure drop may be in error. 

These conditions are similar to flow through orifices, nozzles, and venturi tubes. Flow through nozzles and venturi devices is limited by the critical pressure ratio, r,. = downstream pressure/upstream pressure at sonic conditions (see Figure 2-38C). 

Orifice plates may be treated in the same way as valves using an appropriate value of flow control characteristic within acceptable cavitation limits. 

The variable orifice of the expansion valve can be replaced, in small systems, by a long thin tube. This is a non-modulating device and has certain limitations, but will give reasonably effective control over a wide range of conditions if correctly selected and applied. Mass flow is a function of pressure difference and the degree of liquid subcooling on entry. 

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