Positive Displacement Pumps and Reciprocating Pumps

Positive Displacement Pumps

• These pumps alter the volume of fluid in the cylinder from maximum to minimum in each cycle.
• The volume increases from a small to large space, then decreases back to small size again.
• Fluid is separated from high-pressure and low-pressure zones to prevent leakage and return to the low-pressure source.
• Displacement of fluid is caused by altering the physical volume of a sealed pumping chamber.
• Fluid volume increases as the chamber moves, then decreases, forcing the fluid outwards.
• The volume of fluid pumped is consistent due to the positive displacement feature of the chamber.

Irregularity Grade

• The ratio of flow variation (Qmax - Qmin) to the average flow rate (Qaverage) is the irregularity grade (G).
• G= (Qmax_Qmin*100) / Qaverage

Reciprocating Pumps

• Widely used in oil rig towers and boiler feed units in steam-powered companies.
• Used in chemical companies, especially those made of PVC (as it resists corrosion from acids and bases).
• Made of cast iron (except high-pressure pumps, which are made of steel). Some are also made of chromium-molybdenum-steel alloys for higher pressures.
• Pistons are either disc-shaped or cylindrical (cylindrical pistons).
• Cast iron is often used in pistons to reduce friction.
• Small pistons are often made from a single block, while larger ones (over 10 cm) are made from steel.

Valves

• Valves facilitate fluid intake and discharge.
• Light-weight valves prevent hydraulic shocks during closure.
• Self-acting (automatic) valves open due to increased pressure beneath.
• Ball valves consist of a body, ball (often solid or hollow—bronze or steel), cover, and valve seat. Useful for viscous and oily fluids.

Piston Rod

• The piston rod is positioned between the piston and the crankshaft.
• Translates rotational motion to linear reciprocating motion.
• Made from high-quality alloy steel containing chromium and nickel, sometimes aluminum alloys.
• Manufactured through hot forging and heat treated

Crank Shaft

• Receives rotational motion from the piston, changing it into linear reciprocating motion.
• Made through hot forging and heat-treated to eliminate internal stresses resulting from the forging process.
• A carbonization or electric furnace process is used for the external surface hardening for corrosion resistance.

Packing's

• Rings made of asbestos coated with graphite, preventing fluid leakage and air entry.
• Types depend on fluid properties (nature, pressure, temperature, hazards).
• Soft packing: Used when atmospheric pressure is higher than the cylinder pressure.
• Mechanical seal: Type of packing used for high pressure or high temperature conditions.

Lantern Ring

• Disc-shaped ring with holes for distributing coolant material on the asbestos ring.

Methods of Power Transmission

• Direct (using couplings).
• Indirect (chains, belts, gearboxes).

Pump Speed

• Relationship between the rotational speed of the crankshaft and the linear speed of the piston.
• v = ω.r.Sin a
• ω = (2πn)/60

Reciprocating Pump Types (Single Action, Double Action, Multi Piston)

• Single action, single piston pumps: Fluid flows intermittently in one direction with a complete crankshaft cycle.
• Double action, single piston pumps: Fluid flows continuously in both directions within a half-crankshaft cycle.
• High irregularity grade in single action pumps.

Air Chamber

• Used to stabilize fluid flow in reciprocating pumps.
• Acts as a buffer against pressure fluctuations.
• It is a reservoir with a capacity 6-9 times the cylinder's displacement volume.

Strainer (Filter)

• Prevents impurities and debris from entering the pump.
• Multiple types exist (circular, cylindrical, conical, disc-shaped).

Foot Valve

• Ensures fluid presence in the suction pipe and expels trapped air.
• Used when the supply source is below pump level.

Relief Valve

• Automatically releases pressure above a predefined limit, protecting downstream equipment.
• Not suitable for systems handling air, steam, gases, or volatile materials.

Gear Pumps (External and Internal)

• External gear pumps: Two gears rotate in opposite directions causing enclosed compartments that push the fluid out.
• Internal gear pumps: Internal gear drives the external gearing.

Screw Pumps

• Reliable and low noise, suitable for many applications.
• Fluid is displaced axially with the screw's rotation.

Vane Pumps (Single and Double Action)

• Centrifugal force moves vanes, creating a pumping action.
• Constant displacement; flow rate altered by engine speed changes.

Diaphragm Pumps

• Fluid and air compartments are separated by pliable diaphragms.
• Movement of air into/out of the air compartments controls fluid displacement.
• Single diaphram or double diaphragm.

Centrifugal Pumps

• Utilize centrifugal force to lift fluids.
• Simple designs, high adaptability.
• Consist of an impeller rotating within a volute casing.
• Fluid enters the center of the impeller, accelerated, and expelled to the casing's periphery.
• Constant flow is ensured by the impeller's function.

Diffuser Type Pumps

• The impeller is surrounded by stationary, gradually expanding vanes.
• The diffuser converts kinetic energy to pressure.

Stuffing Box or Shaft Seal

• Enclosures to prevent fluid leakage from the pump shaft.
• Static (gasket-type seals used when no relative motion occurs) or dynamic (soft packing, mechanical seals—used when motion is present).

Buffer

• Used in mechanical seals.
• Reduces vibration.

هذا الاختبار يتناول مضخات الإزاحة الإيجابية ومضخات التباديل. تعلم كيفية عمل هذه المضخات، وكيفية قياس عدم انتظام التدفق. يركز الاختبار أيضًا على تطبيقاتها في الصناعات المختلفة مثل النفط والكيمياء.