Piston pumps delivery from

The actual cylinders and pistons of a two-headed pump are constructed in a very similar manner to the single piston pump with a sapphire piston and a stainless steel cylinder. Each cylinder is fitted with nonreturn valves both at the inlet and outlet. The cams that drive the two pistons are carefully cut to provide an increase in flow from one pump while the other pump is being filled to compensate for the loss of delivery during the refill process and thus, a fall in pressure. A diagram of a twin-headed pump is shown in figure 10. 

Traditional Apparatus. As indicated earlier, liquid delivery systems for controlled rate addition of monomers and initiators have tended to rely upon constant speed piston pumps (19) in which volumetric control is achieved by manual adjustment of stroke length, and monitoring is by discharge from measuring cylinders. 

Many of the applications being developed for responsive gels are for controlled drug delivery, as reviewed by Gehrke and Lee, Hoffinan, Tanaka and Urry [1,28,87,88]. There are several ways in which these devices might respond to an appropriate stimulus. They could expand like pistons to pump drug from reservoirs, contract around reservoirs to squeeze out drug solutions, or function... 

In the MiniMed implantable pump, a piston pump drives insulin through the delivery catheter. A patented solenoid motor controls the piston movement, to aspirate insulin from the reservoir chamber into the piston chamber and then push it through the insulin delivery catheter. 

Diaphragm, piston, and peristaltic pumps all deliver liquids in pulses. This uneven delivery can cause dosing problems when mixing is not sufflcient to ensure a rapid distribution of the chemical into the process water stream. Devices and systems are available to help smooth the delivery from these types of pumps. These devices are sometimes called pulsation dampeners (Figure 7-6). There are several designs. 

Most pumps that are used in hydraulic applications have a fixed displacement which cannot be changed except by replacing certain components. However, in some, it is possible to vary the size of the pumping chamber and thereby the displacement by means of external controls. Some unbalanced vane pumps and many piston units can be varied from maximum to zero delivery or even to reverse flow without modification to the pump s internal configuration. 

In two-parallel-head pumps the pistons are driven by two identical cam discs having a phase shift of 180°. The liquids displaced from both heads are combined in the outlet manifold assembly to produce a continuous solvent delivery. The two-heads-in-series design is used by most manufacturers. In this mode the second piston displaces a volume that is one-third or one-half of the volume displaced by the first piston (the working piston). During the working piston s forward stroke, the downstream piston is moving backward and its chamber is refilled at high pressure so that the flow rate delivered to the column is constant. 

This consists of a solvent delivery for isocratic reversed phase and gel filtration chromatography. The isocratic system (Fig. 1.1) provides an economic first step into high performance liquid chromatography techniques. The system is built around a high performance, dual-piston, pulse-free pump providing precision flow from 0.01 to 5mL min-1. 

The rate of fluid discharge from the cylinder is zero at the beginning of a piston stroke and increases to a maximum value when the piston reaches full speed. If only one discharge cylinder is used, the flow rate will pulsate. These pulsations can be reduced by placing an air chamber on the discharge line or by using a number of delivery cylinders compounded. Simplex pumps have only one delivery cylinder, duplex pumps have two cylinders, and triplex pumps have three cylinders. 

When a steam pump is used, the pressure of the steam in pounds per unit area times the area of the piston would be the maximum force that could be exerted on the work-delivery piston if the machine were perfect and no friction were involved. However, friction is involved and work must be done on the liquid (or work-receiving fluid) under conditions in which the steam pressure is a finite amount greater than the liquid pressure. The ratio of the pressure theoretically required on the steam piston to the pressure actually exerted by the steam is known as the pressure efficiency or steam-end efficiency. It includes the effects of piston and rod friction, momentum changes in acceleration of the piston and fluid, and leakage of fluid past the piston. The pressure efficiency varies from about 50 percent for small pumps up to 80 percent for large pumps. 

Screw-driven syringe pumps consist of a large syringe in which the piston is moved by a motor-driven screw. They me pulse free, and the rate of delivery is easily varied. They suffer from lack of capacity and are inconvenient when solvents must be changed. 

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