You're push, push, pushin' me away
You're push, push, pushin' me away
You're push, push, pushin' me away

("Pushing Me Away," by the Jonas Brothers, 2008)

FOCUS ON RECIPROCATING-ACTION POSITIVE DISPLACEMENT PUMPS

The nearby chart depicting "The Pump Family Tree … with Examples" shows the Positive Displacement Pump lineage on the right side of the diagram.

Two classifications of Positive Displacement (PD) Pumps are shown on the chart; "Reciprocating-Action PD Pumps" appear on the upper right of the chart and "Rotary Motion PD Pumps" appear on the bottom right.

Brilliant PTOA Readers and Students …

meaning those who read the PTOA Segments in the intended, sequential order …

were first introduced to Reciprocating-Action Positive Displacement Pumps in PTOA Segment #204.

Thus, PTOA Readers and Students already know that a Reciprocating-Action Positive Displacement Pump increases the  PV Pressure energy of a liquid by forcing the liquid into a much smaller volume. The increased PV Pressure equally impacts the internal surfaces the Cylinder where the liquid is squashed.

In other words the liquid is "displaced" by some kind of Displacer.

This PTOA Segment #209 delves more deeply into Reciprocating-Action PD Pumps … aka the Positive Displacement Pumps which have Displacers called Pistons, Plungers, or Diaphragms that are continuously moving with a back-and-forth (or up-and-down) action.

 

TWO "GOOD THINGS" ABOUT RECIPROCATING-ACTION PD PUMPS

The operating differences between PD Pumps and Centrifugal Pumps were featured in PTOA Segment #205.  Two "good things" about PD Pumps can be more fully understood and appreciated after reviewing the many factors that influence Centrifugal Pump Performance optimization.

Characteristic Curves are Centrifugal Pump phenomena; each curve depicts the relationship between the Total Dynamic Head expressed in Feet on the Y axis as the Capacity of the liquid that flows through the pump increases (units are gallons per minute, gpm, on the X axis). PTOA Readers and Students already learned how the Centrifugal Pump's Characteristic Curves are developed in PTOA Segments #166 and #167.

The Centrifugal Pump's dashed, curved line arcs downward as the pump's Capacity increases. Translation:  the Total Dynamic Head arcs downward as the liquid's flowrate through the pump increases.

Reminder! The Total Dynamic Head is just the pump's Discharge PV Pressure … expressed in Feet of Head instead of psi.

In contrast, the red, vertical line of the PD Pump informs the observer that the PD Pump Characteristic "Curve" is not a curve at all but rather a single TDH/Capacity relationship. The PD pump in the nearby graphic churns out a steady Capacity of 75 gpm at a constant 225 TDH (aka Discharge PV Pressure). This Discharge PV Pressure is significantly greater than any magnitude of Discharge PV Pressure attainable by a comparable Centrifugal Pump at any Capacity.

The nearby Centrifugal Pump Performance Curve is familiar to all PTOA Readers and Students because the interpretation and evaluation of Centrifugal Pump Performance Curves was featured in PTOA  Segment #168.

The Centrifugal Pump Performance Curve adds more information to the Centrifugal Pump Characteristic Curve ... for example the Centrifugal Pump's Efficiency Curve.

Thus the nearby Centrifugal Pump Performance Curve clearly shows that …

as the Capacity of the Centrifugal Pump increases and the PV Discharge Pressure arcs downward …

the pump's purple, rising Efficiency Curve increases …  until it starts dropping off in the higher Capacity range.

Hence the goal of every Process Operator is to operate a Centrifugal Pump at the Capacity which is correlates to optimal Pump Efficiency.

Don't shoot the messenger but this tedious exercise of revisiting Centrifugal Pump Characteristic Curves and Performance Curves is to bring home the point that Process Operators DO NOT NEED TO WORRY about operating the Reciprocating-Action PD Pump efficiently!

The inherently high Efficiency of small and large PD Pumps is practically independent of system variations.

As long as there is liquid constantly supplied to the Reciprocating-Action PD Pump, the Displacer (a Piston, Plunger or Diaphragm) is going to "push out" (aka "displace") a constant volume of liquid that has a high PV Discharge Pressure.

Of course the back-and-forth or up-and-down movement of the Displacer originates from some kind of Driver. In the nearby Reciprocating-Action PD Pump gif the Crankshaft might imply an Engine is driving the Pump's Shaft (refer to PTOA Segment #192).

In summary,

A Reciprocating-Action PD Pump will dependably and efficiently discharge the same quantity of liquid (aka Capacity) at a high PV Discharge Pressure limited only by the capabilities of the Driver and the endurance strength of the Pump to withstand all the Axial Thrust that results from constantly reciprocating action.

Because of their capability to deliver:

• A low but constant Capacity (aka Flowrate) of liquid with a high PV Discharge Pressure at a
• Consistently High Operating Efficiency

 

the ideal industrial service application for Reciprocating-Action PD Pumps is wherever a relatively low yet constant flow of liquid with a high PV Pressure is required.

The Triplex Pumps shown in the nearby photo were featured in PTOA Segment #204 and were  an important component of the  Artificial Lift Systems described in PTOA Segment #208 .

The Triplex Pump incorporates Plungers and Diaphragms as Displacers and indeed will be found in process service applications wherein a relatively low flowrate of high PV Pressure liquid is needed.

In this type of service the significantly higher installation costs of the Reciprocating-Action PD Pump will be offset by the consistently high operating Efficiency.

 

 

Anatomy of "Single Acting" Reciprocating-Action Piston Positive Displacement Pumps

Some of the important hardware found in a "Single Action" Reciprocating-Action Piston PD Pump is labelled in the nearby gif:

• A Suction Valve and Discharge Valve have synchronized action that allows the liquid being pumped to enter the (not labelled) Cylinder via the Suction Valve and thence discharge the squashed, high PV Pressure liquid through the Discharge Valve.
• Although it is not shown on the gif, a Suction Side Check Valve and a Discharge Side Check Valve make certain the liquid flows through the pump as planned and that no backflow occurs. PTOA Readers will learn about the form and function of Check Valves in the upcoming PTOA PV Flowrate Focus Study Area so do no stress about that now.
• A Piston, Crosshead and Connecting Rod are continuously pushed back and forth because the Connecting Rod translates the rotary motion of the Driver into back and forth motion.
• The Driver for the Pump is not defined. The Crankshaft hints at an Engine, but all PTOA Readers and Students know that the rotary motion and power of
  

  Pascal's Law: When a confined fluid … like a liquid … is pressurized, that PV Pressure is transmitted in all directions in the container (like a Cylinder!)

  a Steam Turbine (PTOA Segment #193) or a Motor (PTOA Segments #187 and #188) could be converted into back and forth (or up and down) action by the Crankshaft as well.

• The back and forth movement of the Piston displaces … or forcibly pushes … the liquid into a much smaller volume. Although liquids cannot be compressed like gases, PTOA Readers and Students learned in PTOA Segment #147 that Pascal's Law explains how hydraulics can be used to create significant PV Pressures and how the Force component of that PV Pressure can be used to do work! Eventually the increased PV Pressure of the liquid squashed in the Cylinder triggers the Discharge Valve to open and the discharge liquid flows into a header.

 

PTOA Readers and Students should immediately recognize that the PD Pump must have Packing  and a Packing Gland encased in a Stuffing/Packing Box to prevent leakage of the pumped up fluid into the surroundings as well as Seals to prevent the surrounding atmosphere from contaminating the pump internals. The form and function of Stuffing/Packing Boxes was featured in PTOA Segments #181.

Likewise, the Axial Thrust of the Shaft must be countered with Bearings (PTOA Segments #182 through #184)

More modern Triplex Reciprocating-Action PD Pumps like the one shown in the nearby gif have totally enclosed, self lubricating power ends and are thus protected from leaked pumped liquid or dirt from the surrounding atmosphere.

 

 

 

"DOUBLE ACTING" RECIPROCATING-ACTION PISTON PUMPS

As stated above, the constantly thrusting Axial Force generated by the Piston in a Single Acting" Reciprocating-Action Piston PD Pump requires mega Bearings to offset.

The "Double Acting" Reciprocating-Action PD Pump balances the forward/backward movement …  and thus Axial Thrust … because the stroke of the single Cylinder extends and contracts between TWO duo-duty Suction/Discharge Valves as shown in the nearby gif.

During the "Extension Stroke" the pumped-up liquid located in the left side of the pump is pushed out of the left side duo-duty Suction/Discharge Valve.Simultaneously, liquid flows into the duo duty Suction/Discharge Valve on the right side of the pump's Cylinder.

During the "Contraction Stroke" the backward movement of the Piston pushes the pumped up liquid on the right side of the Cylinder out of the duo-duty Suction/Discharge Valve. Simultaneously, liquid flows into the left side of the Cylinder through the duo-duty Suction/Discharge Valve on the left side of the Pump.

"Double Acting" Reciprocating-Action PD Pumps deliver twice as much pumped-up liquid Capacity with the same Driver output and are thus much more Efficient than a "Single Acting" Reciprocating-Action PD Pumps with same size Cylinder-Piston arrangement. Additionally, the balanced Axial Thrust results in much less wear and tear on the pump.

 

 

HOW TO DETERMINE THE VOLUMETRIC EFFICIENCY OF ANY TYPE OF PUMP

The information in this section is pertinent to all kinds of pumps, not just Reciprocating-Action Positive Displacement Pumps.

Unlike pumps used in a household dishwasher or clothes washer or in an automobile, industrial processing pumps are not "stock pumps" but rather custom ordered from a Pump Manufacturer for a specific industrial pumping service.

The Pump Name Plate reveals the results of pre-shipment testing and gives the Process Operator an idea of what the capabilities of the Pump are … or were before any modifications took place.

The Pump Name Plate will be securely attached to the pump. Pump Name Plates are crucially important when a facility is being built because dozens if not hundreds of pumps will need to be installed in the right place.

As the nearby graphic of a Sulzer Bingham Pump Name Plate indicates, the size of a pump can be expressed via the following parameters:

• Capacity (like gpm)
• Discharge Pressure in the form of TDH and abbreviated "HEAD"
• Speed (rpm) of Shaft Rotation
• Impeller Size for a Centrifugal Pumps or
• Cylinder Size for a Reciprocating-Action PD Pump

 

The nearby picture of a Sulzer Bingham Centrifugal Pump Name Plate indicates this Pump is rated to pump a liquid with a Specific Gravity of 0.639 at:

• A Capacity of 40 U.S. gallons per minute and at a
• A Total Discharge Head of 112 Ft (multiply by 2.31 = 259 psi ...see PTOA #166)
• The Impeller size is 7.5 inch
•  The Shaft Speed is 3560 rpm (hard to read!)

 

In the real world Pump Name Plates get quite worn out.

The nearby plate for an Ingersoll-Dresser Pump indicates that this pump is in the service of pumping Crude Oil with a Specific Gravity of 0.902.

The Name Plate for the Ingersoll-Dresser Crude Oil Pump reveals it is "sized" to pump 900 US gallons per minute (gpm) at a Head of 990 Feet (multiply by 2.31 = 2287 psi) as the Shaft is spinning at 3540 rpm.

The Ebara Pump Name Plate shown nearby is for a Positive Displacement Pump. This pump delivers a constant 2 cubic meters per minute (m³/min) at a Head of 20 meters.

Process Operators in the USA, Burma, and Liberia would need to convert to the British Imperial Units of Measurement using the factor-label conversion method featured in PTOA Segment #148 to determine that this Ebara Pump has a Capacity of 528 gpm at a Head of 65.6 Feet (multiply by 2.31 = 1515 psi).

 

Volumetric Efficiency of a Pump

Assume that all three of the pumps are installed and while operating in the Real World it is determined that:

• The Sulzer-Bingham Pump delivers 30 gpm … not 40 gpm.
• The Ingersoll-Dresser Pump delivers 700 gpm … not 900 gpm.
• The Ebara PD Pump delivers 425 gpm  … not 528 gpm.

 

It is not uncommon to find that the Real World performance of any pump does not match the results observed within the pristine environment of the Pump Manufacturer's testing facilities.

There are several reasons for the difference, but the one that is of most interest to Mechanics and Mechanic Techs is too much clearance than desired between the pump's internal hardware. For example, too much gap between the Impellers and Volute of a Centrifugal Pump or too much clearance in the meshed gears of a Rotary-Motion Gear Pump.

Any volume of liquid that is not impacted by a swirling Impeller or squashed by some type of Displacer will decrease the volume of liquid (aka Capacity) that is infused with PV Pressure energy.

The mathematical expression that defines the Volumetric Efficiency of a Pump in Percent is:

(Real World Observed Capacity ÷ Name Plate Capacity) X 100

The Volumetric Efficiency of the Pumps with the above Name Plates are:

• Sulzer-Bingham Pump    30 gpm/45 gpm  X 100 = 75%
• Ingersoll-Dresser Pump :  700 gpm/900 gpm X 100 = 77%
• Ebara PD Pump:  450 gpm / 528 gpm X 100 = 85%

 

Pump Name Plates Can Help Estimate Changes in Rotary-Motion PD Pump Capacity 

Pump Name Plates can also be used to predict expected changes in Rotary-Motion PD Pump Capacity.

For example, assume the Pump Name Plate for a Rotary-Motion Vane Pump states that the pump is rated for a 6 gpm Capacity at 1200 rpm.

Next, assume that more pump Capacity is needed so the decision is made to increase the Driver speed such that the Vanes rotate at 1800 rpm.

What Capacity would be predicted if the Driver were changed from 1200 rpm to 1800 rpm?

Hey! This is one of those basic ratio calculations from elementary school math! If 1200 rpm yields 6 gpm, how much more gpm does 1800 rpm yield?

(1800 rpm / 1200 rpm) * 6 gpm = 9 gpm

Thus, on paper, the upward adjustment on the rotational speed of the Vanes would be expected to increase Capacity to 9 gpm.  The Real World outcome might be different, but for a rough estimate the Pump Name Plate information is a good starting point.

 

TAKE HOME MESSAGES: There are two classifications of Positive Displacement (PD) Pumps, Reciprocating-Action PD Pumps and Rotary-Motion PD Pumps.  This PTOA Segment and the next PTOA Segment feature Reciprocating-Action PD Pumps.

Reciprocating-Action PD Pumps use Displacers (Pistons, Plungers, and Diaphragms) to push liquids into a smaller volume until the PV Pressure of the liquid has been hydraulically increased with sufficient PV Pressure energy to trigger a Discharge Valve to open.

For a Centrifugal Pump, the Total Discharge Head-Capacity-Operating Efficiency relationship of a Centrifugal Pump varies. 

In contrast, the Reciprocating-Action PD Pump has one Capacity-Discharge Pressure relationship and dependably high operating Efficiency.  For a pumping service that requires a consist low-range flowrate discharged at a constant high PV Discharge Pressure, the consistently high operating Efficiency of a Reciprocating-Action PD Pump will offset the significantly higher investment cost.

The crucial hardware of a Reciprocating Action pump includes:

• Cylinders with Suction Valves and Discharge Valves.
• Suction Check Valve and Discharge Check Valve.
• Displacer (Piston, Plunger, or Diaphragm) and Connecting Rod attached to a Crosshead.
• Crankshaft and Driver which continuously extend and contract the Displacer within the Cylinder. 

 

PD Pumps have Stuffing Boxes, Seals, and Bearings.

Compared to a similarly sized "Single-Acting" Reciprocating-Action PD Pump, the "Double-Acting" Reciprocating-Action PD Pumps has twice as much Capacity/Flowrate discharged from the Pump per each stroke of a Displacer with no sacrifice in Discharge Pressure. Thus the "Double-Acting" Reciprocating-Action PD Pump is much more Efficient and the Axial Thrust is balanced which causes less wear and tear on the pump.

Pump Name Plates are attached to each industrial pump. Pump Name Plates reveal important pump parameters (e.g. Capacity and Head) that the Pump Manufacturer observed during pre-shipment testing conducted under ideal conditions.

Pump Name Plates can be used to estimate the Volumetric Efficiency of a Pump.

The Volumetric Efficiency of a pump can be greatly impacted by the clearances between pump internals. Too much space between pump internals will decrease the Capacity of a pump.

The percent Volumetric Efficiency of any Pump (not just PD) can be estimated: (Real World Observed Capacity ÷ Name Plate Capacity) X 100

For Rotary-Motion PD Pumps, Pump Name Plates can be helpful predicting the Capacity change that would result when the Driver speed is changed.  

 

©2020 PTOA Segment 0209
PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT PUM

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