A General Guide to Positive Displacement Pumps - WittyWriter

A General Guide to Positive Displacement Pumps

1.0 Introduction

This guide provides general principles for the selection and application of positive displacement (PD) pumps. It covers pump types, characteristics, accessories, and critical overpressure protection requirements. For heavy-duty refinery or petrochemical services, pumps should typically comply with standards such as API-676 for rotary positive displacement pumps.

2.0 Pump Selection

Centrifugal vs. Positive Displacement Pumps

The choice between a centrifugal and a positive displacement pump depends on the application. PD pumps are often the better choice in the following scenarios:

Key Application Drivers for PD Pumps

High Viscosity: PD pumps excel at handling viscous fluids (e.g., > 300 cSt) where centrifugal pumps become very inefficient.
Variable Discharge Pressure: A PD pump delivers a near-constant flow regardless of system pressure, making it ideal for metering and dosing. A centrifugal pump's flow would vary significantly with pressure changes.
High Pressure: PD pumps can generate very high pressures, often far more than centrifugal pumps.
Suction Lift: PD pumps create a strong vacuum and are self-priming, allowing them to lift liquids from a lower level (e.g., 8-9 meters). Centrifugal pumps must be primed and have limited suction lift.
Sealing: The slower operating speeds of PD pumps often mean mechanical seals last longer and may not require external flushing plans.
Shear-Sensitive Fluids: The low-speed, gentle action of a PD pump is ideal for fluids that can be damaged by the high-speed shearing of a centrifugal pump (e.g., polymers, food products).

Types of Positive Displacement Pumps

PD pumps are broadly classified into two main types:

1. Rotary Pumps

Gear Pump (Internal/External): Provides a non-pulsating flow and has high-pressure capability. Not suitable for solids. Often used for fuel oil.
Lobe Pump: Can pass medium-sized solids and has no metal-to-metal contact. Good for clean-in-place (CIP) applications.
Vane Pump: Suitable for thin (low-viscosity) liquids but not for high pressures or abrasive solids. Often used in LPG applications.
Progressive Cavity (Single-Screw): Excellent for slurries and viscous fluids but cannot be run dry.
Screw Pump: Offers high flow rates and high-pressure capability but is not suited for thin liquids or slurries.

2. Reciprocating Pumps

Piston/Plunger Pump: Offers very high-pressure capability for metering. Flow is pulsating (multiplex pumps with multiple plungers reduce this).
Diaphragm Pump (Hydraulically Actuated): Provides non-pulsating flow, low NPSH requirement, and is easy to maintain. Used in metering applications.
Diaphragm Pump (Air-Operated, AODD): Portable, can handle solids and abrasives, and requires no shaft sealing. Flow is pulsating, and discharge pressure is limited by the air supply pressure. Often used for barrel unloading or sump emptying.

3.0 Pump Characteristics

Characteristic Curve

A positive displacement pump moves a fixed volume of fluid with each rotation or stroke. As a result, its flow rate is directly proportional to its speed and is nearly constant, regardless of the discharge pressure. This is why a PD pump's characteristic curve is a near-vertical line, whereas a centrifugal pump's curve slopes downward. This behavior is also what creates a significant hazard: if the discharge is blocked, the pump will continue to try to move the fluid, increasing pressure indefinitely until the motor stalls, the piping ruptures, or the pump itself fails.

Net Positive Suction Head (NPSH)

NPSH Available (NPSHA) for a PD pump is calculated similarly to a centrifugal pump, but with one critical addition: Acceleration Head ($H_a$).

Reciprocating pumps create a pulsating flow, which means the entire column of liquid in the suction pipe must be constantly accelerated and decelerated. The energy required for this acceleration is the "acceleration head," which reduces the available NPSH at the pump inlet. This value must be subtracted from the steady-state NPSHA.

Estimating Acceleration Head Loss

A common formula for acceleration head is:

H_a = K × L × V × N × C

Where:

$H_a$ = Acceleration head (in meters)
K = Factor for the fluid (e.g., ~0.05 for hydrocarbons, ~0.10 for liquids with entrained gas)
L = Actual length of the suction line (in meters)
V = Liquid velocity in the suction line (in m/s)
N = Pump speed (in RPM)
C = Constant based on pump type (e.g., 0.200 for duplex single-acting, 0.066 for triplex)

If the effective NPSHA (NPSHA - $H_a$) is insufficient, the pump will cavitate, leading to loss of efficiency and potential damage.

4.0 Pump Accessories and Control

Common Accessories

Pulsation Damper: Essential for reciprocating pumps to reduce pressure pulsations in the discharge line. A damper on the suction side can also reduce acceleration head, improving NPSH conditions.
Diaphragm Failure Indication: For double-diaphragm pumps in process service, a failure switch is used to detect a leak in the primary diaphragm, preventing contamination between the process fluid and the hydraulic fluid.
Calibration Facility: For metering pumps, a graduated calibration pot is often installed on the suction side to verify the pump's flow rate under operating conditions.
Check Valve: Required on the discharge of rotary pumps to prevent backflow. Not typically needed for reciprocating pumps, as their internal valves serve this function.

Flow Control Methods

Since a PD pump's flow is not controlled by a discharge throttle valve, one of these methods is used:

Variation of Stroke Length: The most common method for metering pumps. The length of the plunger's stroke is adjusted (manually or automatically) to change the displaced volume.
Variation of Pump Speed: Using a variable speed drive (VSD) to change the pump's RPM, which directly changes the flow rate. Common for rotary pumps.
Back Pressure Control: A back-pressure control valve on a bypass line routes excess flow back to the pump's suction or source tank. This is common in ring-main systems (e.g., fuel oil supply) that need constant pressure.

5.0 Critical Safety: Overpressure Protection

Because a PD pump will deliver its rated flow regardless of pressure, it must be protected from a blocked discharge. This is the most critical safety consideration for any PD pump installation.

Warning: Do Not Rely on Internal Relief Valves

Many pumps are offered with small, *internal* relief valves. These valves are not designed for process safety and are intended only for brief, momentary protection.

They must not be used as the primary overpressure protection device because they typically do not meet process safety standards:

They cannot be removed for periodic testing.
Their set points are often not repeatable.
Their flow capacity is not certified for full pump flow.
They are not code-stamped for protecting a pressure vessel (if the piping system is code-stamped).

An external, full-flow relief device is always required.

External Overpressure Protection Methods

The external relief system must be capable of relieving the full rated capacity of the pump.

Relief Valve (RV) to Pump Suction: The most common method. The RV discharge is piped back to the pump's suction line.
Caution: If the fluid is near its vapor pressure, this loop can heat up and cause the liquid to flash. A cooler or temperature detector may be needed.
Relief Valve (RV) to Source: A safer alternative. The RV discharges back to the main source vessel (tank). This allows the heat to dissipate and prevents overheating the fluid at the pump suction.
Rupture Discs: Can be used for slurries that would clog a standard relief valve.
Hydraulic Circuit Relief (Diaphragm Pumps): A relief valve in the hydraulic drive fluid protects the pump by stopping the diaphragm's stroke.
Pressure Switch Interlock: A high-pressure switch on the discharge that is interlocked to shut down the pump motor. This is less reliable than an RV and should be backed up by one.
Torque Limiting Device: A device like a shear pin that mechanically disconnects the drive if the pump seizes or overpressures.

Best Practices for Relief Devices

✓ The relief device must be sized for 100% of the pump's rated capacity at the relief pressure.
✓ For fluids that can congeal or solidify (e.g., fluids with a high pour point), the relief valve and its inlet/outlet piping must be heat-traced to ensure it does not plug.
✓ For fluids containing solids, a continuous flush may be needed to keep the relief device lines clear.

6.0 Modifying Pumping Systems

Debottlenecking

Unlike centrifugal pumps, modifying a PD pump's performance is limited. Changing the pump's speed directly changes its capacity but has no effect on its pressure capability (which is set by the system resistance and relief valve). Doubling a PD pump's speed will double its flow and roughly double its power requirement.

Parallel and Series Operation

Parallel Operation: Two PD pumps in parallel will add their flow rates at the same pressure, just like centrifugal pumps.
Series Operation: PD pumps are generally not operated in series, as a single pump can almost always be selected to develop the required discharge pressure.