Positive pressure pumps, types

An infusion control device (ICD) is a device that maintains a constant infusion rate in a gravity flow system (controller) or via a positive pressure pump. A positive pressure pump is a device that provides mechanical pressure (2-12 psi) to overcome the resistance to flow in the vessels. The types of positive pressure pumps are categorized according to how they deliver the solution and their degree of precision in the flow rate. Positive pressure pumps include peristaltic pumps, cassette pumps, syringe pumps, non-electiic or disposable pumps, and patient-controlled analgesic... 

Reverse Osmosis. Membranes are used for the separation of smaller components (<500 daltons). They have smaller pore space and are tighter than those used for ultrafiltration. High pressure pumps, usually of the positive piston or multistage centrifugal type, provide pressures up to 4.14 MPa (600 psi). 

Positive Displacement Pumps. Positive displacement pumps foUow HI convention (see Fig. 1). As a rule, these pumps work against significantly higher pressures and lower flows than do kinetic, particularly centrifugal, pumps. Positive displacement pumps also operate at lower rotational speeds. There are many types of positive displacement pumps, for which designs are constantly being developed. Some of these are discussed herein. 

Rotary Pumps In rotary pumps the liquid is displaced by rotation of one or more members within a stationary housing. Because internal clearances, although minute, are a necessity in all but a few special types, capacity decreases somewhat with increasing pump differentia pressure. Therefore, these pumps are not truly positive-displacement pumps. However, for many other reasons they are considered as such. 

For positive displacement pumps, a bypass-type control valve should be furnished to set the primary lube system pressure. The valve should be able to maintain system pressure during pump startup and pump transfers, which includes relieving the capacity of one pump, while both are running. The valve should provide stable, constant pressure during these transients. Flow turndown of 8 to 1 is not unusual. Multiple valves in parallel should be used if a single valve is not suitable. The valve should be sized to operate between 10 and 90% of the flow coefficient (Cv). Additional pressure control valves should be furnished as required to pro ide any of the intermediate pressure levels. 

A useful summary of the typical equipment used for developing and maintaining process system vacuum is presented in Table 6-1. Also see Birgenheier [33]. The positive displacement type vacuum pumps can handle an overload in capacity and still maintain essentially the same pressure (vacuum), while the ejectors are much more limited in this performance and cannot maintain the vacuum. The liquid ring unit is more like the positive displacement pump, but it does develop increased suction pressure (higher vacuum) when the inlet load is increased at tlie lower end of the pressure performance curve. The shapes of these performance curves is important in evaluating the system flexibility. See later discussion. 

With the exception of piston-type pumps, most of the common positive-displacement pumps utilize rotating elements to provide a constant-volume, constant-pressure output. As a result, these pumps can be monitored with the following parameters hydraulic instability, passing frequencies, and running speed. 

Positive-displacement pumps can be divided into two major types rotary and reciprocating. All rotary pumps use some form of rotating element, such as gears, vanes, or lobes to increase the discharge pressure. Reciprocating pumps use pistons or wobble plates to increase the pressure. 

A final aspect of GPC solvent delivery relates to the solvent reservoirs themselves. The ability to perform in situ helium degassing of solvents, provide inert gas blankets over solvents, and protect solvents from contamination from external sources are worth consideration from the standpoints of convenience and safety alone. If these features are provided for, it is a small step to also provide a small positive pressure, say 10 psi or so, to the solvent reservoir. This positive pressure helps minimize the formation of solvent vapors in the pump chamber during the refill part of the pump stroke, and improves the flow rate reproducibility of rapid-refill type pumps delivering high-vapor-pressure solvents. 

A propeller pump is another type of positive displacement pump. It operates similar to a fan blade inside of a pipe or tube. As fluid moves past the blade, the energy of the moving blade delivers fluid through the outlet at a higher pressure than delivered. 

A useful feature of the PD pump is that for a given power input Eqs. (159)—(161) allow the designer considerable flexibility in adjusting discharge pressure, cylinder capacity, and overall capacity. Positive displacement pumps are favorites on large-scale, high-pressure systems. Details of each of the various types of PD pump are best obtained from individual vendors. 

The other type of constant flow pump is a positive displacement syringe. It is pulseless but suffers from the limited volume it can deliver before refilling. The major type of constant pressure pump is a pneumatically driven syringe. 

For transferring water, a positive displacement, screw type pump is necessary centrifugal pumps will introduce cavitation when they have either a restricted imput or output stream. Even with a screw type pump, it is advisable to insert a bubble trap between the pump and the tube. Such a trap consists of a steel cylinder, perhaps a foot high, in which the water is pumped into near the top and extracted from the bottom. Bubbles will rise to the top where the air they carry can be removed by a valve on the cylinder. In addition, perhaps due to the wall thickness of 1/2 , we have found it necessary to pressurize the tube to 100 psi to obtain accurate data. Bath clear (VWR Scientific, Inc.) or iodine can be added to the water to control bacterial growth. 

Chapter 5 considered pump types and their evaluation and selection. After selecting a pump type, the next step is to size the pump. This requires calculating the flow rate and the pressure rise across the pump or the pump head. The net positive suction head (NPSH), is also important, particularly for centrifugal pumps. NPSH is the difference between the total pressure and the vapor pressure of the fluid at the pump inlet. NPSH will be discussed later. 

Depending on the designed maximum product pressure of the downstream scraped-surface heat exchanger and the various types of margarine produced, high-pressure positive-displacement pumps with maximum discharge pressures of 40 bars (about 600 psi), 70 bars (about 1030 psi), or 120 bars (about ISOOpsi) are normally installed in the process hne. 

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