12.2 Positive Displacement Pumps, Priming & Ancillary Mechanical Equipment

12.2 Positive Displacement Pumps, Priming & Ancillary Mechanical Equipment

1. Positive Displacement (PD) Pumps: Types, Mechanics & Safety

Unlike dynamic centrifugal pumps, positive displacement pumps move fluid by trapping a fixed, specific geometric volume of liquid within a chamber and physically displacing (forcing) that volume through the discharge check valve.

                               Centrifugal Pump vs Positive Displacement Pump
    Head (H)                                     Discharge Flow (Q)
       │  Centrifugal: Flow drops as Head rises     │   Positive Displacement: Flow is CONSTANT
       │  ╲                                         │   regardless of Head (Q ∝ RPM)
       │    ╲                                       │   │
       │      ╲                                     │   │
       │        ╲                                   │   │  (Deadheading builds infinite pressure!)
       │          ╲                                 │   │
       └──────────────────── Flow (Q)               └───┴───────────────── Head (H)

Fundamental Hydraulic Distinction

• Constant Volume Delivery: The volumetric flow rate of a positive displacement pump is directly proportional to rotational or reciprocating speed ($Q \propto RPM$) and is virtually independent of the discharge system head.
• The Deadhead Danger: If a centrifugal pump discharge valve is closed while running, the impeller merely spins ("slips") inside the trapped casing volume, generating its rated shutoff head without building infinite pressure. In stark contrast, if a positive displacement pump is operated against a closed discharge valve (deadheaded), the machine will continue forcing fluid into the blocked line with every stroke. Discharge pressure will climb exponentially within milliseconds until the electric motor stalls, the pump casing fractures, the drive shaft snaps, or downstream piping violently ruptures.

CRITICAL MANDATORY SAFETY REQUIREMENT: Pressure Relief Valves

Non-Negotiable Safety Rule: Every positive displacement pump installation MUST be equipped with an in-line Pressure Relief Valve (PRV) installed on the discharge piping UPSTREAM of any shutoff, check, or isolation valve. The relief valve discharge must be piped back to the supply wet well or suction reservoir, never to atmosphere where operators could be sprayed. Rupture discs or motor thermal-overload switches are auxiliary protections and can never substitute for a properly sized, calibrated hydraulic pressure relief valve.

         ┌──────────────────────────────┐
         │  Positive Displacement Pump  │
         └──────────────┬───────────────┘
                        │
                        ▼
             [ Discharge Piping ]
                  │           │
                  │           ├──► [ Isolation Valve ] ──► System Main
                  │           │
                  ▼           ▼
            ┌───────────┐   (If closed, PRV prevents explosion!)
            │ Pressure  │
            │  Relief   │
            │ Valve(PRV)│
            └─────┬─────┘
                  │
                  ▼
      [ Piped Directly Back to Suction Well ]

Types of Positive Displacement Pumps in Utilities

+-----------------------+-----------------------------+--------------------+-----------------------------+
| Pump Classification   | Operating Mechanism         | Sludge Solids %    | Critical Operational Rule   |
+-----------------------+-----------------------------+--------------------+-----------------------------+
| Progressive Cavity    | Single helical tool-steel   | 3.0% - 15%+ TS     | MUST NEVER RUN DRY;         |
| (Moyno / Seepex)      | rotor turning in elastomer  | (Dewatered cake,   | stator tears from friction  |
|                       | double-internal-thread      | thickened sludges) | within seconds without fluid|
+-----------------------+-----------------------------+--------------------+-----------------------------+
| Peristaltic           | Rotating shoes/rollers      | 1.0% - 10% TS      | Hose is only wear part;     |
| (Hose Pump)           | compress reinforced hose    | (Chemical slurries,| monitor hose lubrication    |
|                       | inside circular housing     | lime, hypochlorite)| bath for rupture fluid      |
+-----------------------+-----------------------------+--------------------+-----------------------------+
| Air-Operated Double-  | Compressed air shuttles two | 0.5% - 8.0% TS     | Stalls safely if deadheaded;|
| Diaphragm (AODD)      | diaphragms connected by     | (Scum, sumps, oily | can run dry indefinitely;   |
|                       | shaft with ball check valves| wastes, chemicals) | requires clean air supply   |
+-----------------------+-----------------------------+--------------------+-----------------------------+
| Reciprocating Plunger | Heavy motor-driven piston   | 4.0% - 10% TS      | Pulsating flow; requires    |
| / Piston Pump         | displaces slurry through    | (Raw primary dense | pulsation dampeners; check  |
|                       | suction/discharge ball seats| sludge, grit)      | ball seats for wedged debris|
+-----------------------+-----------------------------+--------------------+-----------------------------+

1. Progressive Cavity (PC) Pumps:
   • Consists of a hard-chrome plated, single-helix tool steel rotor that turns eccentrically within an elastomeric (Buna-N, Viton, or EPDM) stator molded with a double-internal helix.
   • As the rotor turns, progressing sealed cavities form at the suction port and travel axially toward the discharge port. Fluid is gently conveyed without shearing, making PC pumps ideal for polymers, high-solids sludges (up to 15% TS), and centrifuge feed.
   • The Dry-Run Hazard: Stators rely entirely on the pumped liquid for lubrication and cooling. Running a progressive cavity pump dry for even 30 to 60 seconds will generate extreme frictional temperatures exceeding 300°F (150°C), scorching, blistering, and tearing the rubber stator. Installations must incorporate thermo-switches (temperature sensors embedded in the stator rubber) or ultrasonic flow switches to immediately interlock and shut down the drive motor upon loss of liquid feed.
2. Peristaltic (Hose) Pumps:
   • Fluid is contained completely inside an engineered multi-ply reinforced elastomeric hose. Rotating shoes or rollers progressively squeeze the hose flat against a circular pump casing, sweeping liquid forward.
   • The fluid touches nothing except the inner liner of the hose. Perfect for off-gassing chemicals (sodium hypochlorite), abrasive lime slurries, and ferric chloride that rapidly ruin mechanical seals. Hose pumps can self-prime up to 25 feet and can run dry without catastrophic thermal damage.
3. Air-Operated Double-Diaphragm (AODD) Pumps:
   • Twin flexible diaphragms connected by a center shaft are reciprocated by an air distribution valve shuttling compressed air between opposing air chambers. Fluid is drawn through suction ball check valves and forced out through discharge ball check valves.
   • Highly versatile: intrinsically safe (no electrical motor), self-priming, capable of handling large suspended solids, and can run dry indefinitely without damage. If the discharge line is closed, the pump simply stalls against the air supply pressure without damaging the pump or downstream pipe.
4. Reciprocating Plunger Pumps:
   • A heavy cast-iron plunger reciprocates inside a packing gland, creating alternating vacuum and pressure strokes inside the liquid cylinder. Heavy lead or rubber-covered ball check valves control flow direction.
   • Capable of developing exceptional pressure (100+ psi) to clear clogged primary sludge lines. Plunger pumps produce severe hydraulic pressure pulsations that require suction and discharge pulsation dampeners (air chambers) to smooth fluid hammer.

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2. Centrifugal Pump Priming Systems & Hydraulics

Centrifugal pumps are not self-priming machines. Unlike positive displacement pumps that can draw a vacuum on air, an open centrifugal impeller cannot compress or displace air because atmospheric air has a density approximately $\frac{1}{800}\text{th}$ that of water.

The Physics of Priming

The total dynamic head developed by a centrifugal pump ($H = \frac{v^2}{2g}$) is independent of fluid density. A pump that develops 100 feet of head will develop 100 feet of water column ($\approx 43.3 \text{ psi}$) when full of water, but only 100 feet of air column ($\approx 0.054 \text{ psi}$) when full of air. This minuscule pressure of 0.054 psi is totally incapable of overcoming atmospheric pressure (14.7 psi) to pull liquid up a suction lift pipe. Therefore, before a centrifugal pump can operate, its casing and suction line must be completely primed (evacuated of all air and flooded with liquid).

                                  PRIMING METHODOLOGY
    ┌───────────────────────────┐ ┌───────────────────────────┐ ┌───────────────────────────┐
    │      Flooded Suction      │ │    Vacuum Priming System   │ │   Foot Valve with External  │
    ├───────────────────────────┤ ├───────────────────────────┤ ├───────────────────────────┤
    │ Pump centerline sits      │ │ Auxiliary liquid-ring     │ │ Spring-loaded check valve │
    │ BELOW water level in tank │ │ vacuum pump evacuates air │ │ at bottom of suction line;│
    │ or wet well. Gravity keeps│ │ from top of casing; float │ │ external water line fills │
    │ casing flooded. Ideal.    │ │ switch starts main pump.  │ │ casing and suction pipe.  │
    └───────────────────────────┘ └───────────────────────────┘ └───────────────────────────┘

1. Flooded Suction (Gravity Primed): The pump is installed physically below the minimum water elevation in the supply wet well or storage tank. When the suction gate valve is opened, gravity forces liquid into the casing, driving air out through an open manual or automatic casing vent valve. This is the most reliable engineering design.
2. Automated Vacuum Priming: When pumps are located on a suction lift (centerline above water surface), an automated vacuum skid (liquid-ring or rotary-vane vacuum pump) connects to the high point of the pump volute through a vacuum priming chamber. The vacuum pump draws air out of the suction pipe and pump casing until liquid rises into the priming chamber. An internal float switch trips, shutting off the vacuum pump and permitting the main centrifugal pump to start.
3. Foot Valve with Priming Chamber: A spring-loaded or flapper foot valve (a one-way check valve equipped with a suction strainer) is submerged at the bottom of the suction intake pipe. An external potable or utility water supply fills the casing and suction pipe until liquid spills from the casing vent petcock, trapping a solid column of water above the foot valve. If the foot valve leaks or is held open by debris, the pump instantly loses prime upon shutdown.

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3. Ancillary Mechanical Equipment: Air Compressors, Blowers, Hoists & Cranes

WPI's equipment content areas explicitly list air compressors, blowers, conveyors, hoists and cranes, valves, and pipes and fittings as ancillary equipment operators must inspect, operate, and maintain. These items rarely get their own chapter in a textbook, which is exactly why they get missed.

Air Compressors

Plant air drives pneumatic valve actuators, air-operated diaphragm pumps, instrument air for transmitters and positioners, air-scour systems on filters, and air-lift pumps.

Core components and operator checks:

• Receiver (air tank): dampens pulsation and provides storage. It is an ASME pressure vessel with a relief valve that must never be blocked, adjusted, or bypassed and that should be manually lifted on a routine schedule.
• Aftercooler and moisture separator: compressing air concentrates water vapor. Drain the receiver daily — accumulated condensate corrodes the tank from the inside and carries water into instrument lines.
• Air dryer: refrigerated or desiccant. Instrument air requires a low pressure dew point to keep moisture from freezing or fouling positioners.
• Intake filter: a clogged filter starves the compressor, raises discharge temperature, and increases oil carryover.
• Unloader and pressure switch: sets the cut-in and cut-out pressures. Short cycling usually means an undersized receiver, a leaking check valve, or a system leak.
• Oil-free vs lubricated: instrument air and any air contacting the process is normally oil-free; lubricated units require oil level checks and scheduled changes.

Exam Trap Alert: Compressed air is not a harmless utility. Never use compressed air to clean clothing or skin, never exceed 30 psi for cleaning nozzles, and always lock out and bleed stored pressure before opening any compressed-air component. A receiver holds hazardous energy exactly the way an electrical panel does.

Blowers

Blowers move large volumes at low pressure for aeration basins, air scour, and odor control. Positive displacement (rotary lobe) blowers deliver constant volume against variable pressure and must have a relief valve because deadheading them will fail the casing or the driver. Multistage centrifugal and high-speed turbo blowers deliver variable volume at nearly constant pressure and are throttled by inlet guide vanes or variable frequency drives. Both require clean inlet filters, correct belt tension or coupling alignment, and monitoring of discharge temperature.

Hoists, Cranes & Conveyors

• Hoists and davit cranes lift submersible pumps, mixers, and bar screen rakes out of wet wells. Inspect the load chain or wire rope, hook throat opening and latch, and confirm the rated capacity tag is legible. Never side-pull a hoist and never exceed the rated load.
• Below-the-hook rigging — slings, shackles, eye bolts — is inspected before every lift and removed from service on any cut, deformation, or missing identification tag.
• Conveyors move screenings, grit, and dewatered cake. Guards over nip points, pull-cord emergency stops along the run, and lockout before clearing a jam are non-negotiable. Most conveyor injuries occur while clearing a jam on an energized machine.
• Valves and piping in the plant air, water, chemical, and sample systems require the same discipline: label every line, support and restrain piping, and verify that materials of construction match the chemical service.

4. Practical Operational Scenarios & Exam Traps

Practical Operational Scenario

A lead operator at a municipal wastewater pumping station is conducting morning checks on three 150-HP centrifugal sewage lift pumps. Pump No. 2 is running, but the operator smells burning odor and observes light white smoke wafting from the stuffing box. Inspection reveals that the packing gland follower is cocked at an angle, the gland nuts are cranked down metal-to-metal, and there is zero liquid leakage exiting the gland.

• Diagnostic Investigation:
  1. The operator recognizes that an over-tightened packing gland has completely starved the packing fibers of cooling and lubricating liquid film.
  2. Frictional temperatures have escalated to the point of scorching and carbonizing the synthetic packing yarns, producing smoke.
  3. If allowed to run, the charred packing will seize and deeply score the stainless steel shaft sleeve, requiring a costly emergency overhaul.
• Immediate Remediation Protocol:
  1. The operator immediately shuts down Pump No. 2 and switches lead operation to Pump No. 3.
  2. The operator allows the stuffing box to cool completely, then evenly loosens both gland follower nuts.
  3. Upon restarting, the operator adjusts the gland follower nuts in small, incremental quarter-turns—allowing 10 to 15 minutes between adjustments for packing relaxation—until a steady, controlled leakage rate of 45 drops per minute (within the 30–60 drop/min standard) is established.
  4. The operator confirms that clean seal water is flowing through the lantern ring at 65 psi (which is 8 psi above the 57 psi pump discharge pressure).

Critical Exam Traps

• Trap 1: Zero Leakage on Compression Packing. Exam questions often test whether a "perfectly adjusted" packing gland should have zero leakage. The correct answer is absolutely not. Compression packing MUST leak 30 to 60 drops per minute to provide continuous cooling and lubrication. Zero leakage guarantees a burned sleeve and ruined packing.
• Trap 2: Starting a Progressive Cavity Pump Dry. Operators must remember that progressive cavity pumps can never run dry. The tool-steel rotor will melt or tear the elastomeric stator within 30 seconds of unlubricated operation.
• Trap 3: Throttling Positive Displacement Pumps. You can throttle the discharge valve of a centrifugal pump to adjust flow. You can NEVER throttle the discharge valve of a positive displacement pump; doing so creates an overpressure condition that will rupture piping or destroy the pump housing unless relieved by a safety valve.
• Trap 4: Wear Ring Clearance Replacement Benchmark. When asked when wear rings should be replaced, remember the rule: when the measured diametrical clearance doubles (100% increase) over the manufacturer's original factory specification.

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