Hydraulics Training - Kemapco

Basic Hydraulics

Basic Pump Hydraulics

Basic Hydraulics

This is an overview of the various aspects of Centrifugal Pump Hydraulic Performance and the Curves used to describe it.

Basic Hydraulics

Upon completion participants will be able to: Identify information shown on Pump Performance Curves. Interpret Information shown. Define Terms “Best Efficiency”, “NPSH”, “Bhp”, etc. Define and Apply Affinity Laws To Specify Speed Changes and Impeller Diameter Cuts.

Basic Hydraulics

Casing Impeller

Single Stage End Suction Pump

Basic Hydraulics

Velocity is the Key: The Impeller accelerates the liquid to a Higher Velocity The Casing converts Velocity to Pressure or Head The Head in the Casing increases as you travel around the perimeter The Larger the Impeller or the Faster the Shaft Speed the more Velocity More Velocity equals higher head

Basic Hydraulics

Cutwater

Basic Hydraulics

Typical Centrifugal Pump Performance BEP TOTAL HEAD

EFFICIENCY POWER NPSHR

FLOW - GPM

Basic Hydraulics

Pressure Pressure = Force per unit area. ◆

◆

Gauge Pressure = Is a measured pressure above or below the surrounding atmospheric pressure (i.e. tire pressure). Absolute Pressure = Is the amount of pressure above an absolute vacuum. The absolute pressure at sea level is about 14.7 psi.

Basic Hydraulics

Pressure

Basic Hydraulics

Pressure

Basic Hydraulics

Head Vs. Pressure ◆

A pump head curve (feet or meters) is used to represent any pumped liquid with a viscosity similar to water

H = V2 2g ◆ Process system losses (feet) K = V2 = (ft / s)2 = ft2/ s2 = ft. 2g ft / s2 ft/ s2 ◆

Elevation in Feet or Meters

Basic Hydraulics

Effect S.G. On Pump Performance

Basic Hydraulics

Effect of Specific Gravity on Static Head

Basic Hydraulics

Pump Performance Equations Brake Horse Power Required - Pump BHP = H X Q X SG 3960 X EFF H = Head in Ft. Q = Flow in GPM SG = Specific Gravity EFF = Efficiency as a Decimal BHP = Brake horse power to pump

Basic Hydraulics

Pump Performance Equations Efficiency of the Pump (EFF) EFF = H X Q X SG 3960 X BHP BHP = Brake horse power to pump

Basic Hydraulics

Gauge Height Correction

Gauge height is the distance between the center of the gauge and the datum, usually the centerline of the suction.

Basic Hydraulics

Pump Performance Curves

Basic Hydraulics Pump Performance Curves

Performance Curves are just a Graphic Representation of How Much Pressure (Head) and Flow a Pump Can Produce along with Other Important Aspects of a Pumps Characteristics

Basic Hydraulics Pump Performance Curves

◆ ◆ ◆ ◆ ◆

Information shown: Pump Model, Size and Speed Head vs Capacity Curve Efficiency of the Unit NPSH Required Approximate Brake Horsepower

Basic Hydraulics Typical Pump Performance Curve

Basic Hydraulics Typical Pump Performance Curve Note Title Block

Basic Hydraulics Typical Pump Performance Curve

Head vs Capacity

Basic Hydraulics Typical Pump Performance Curve

Iso - Efficiency Lines

Basic Hydraulics Typical Pump Performance Curve

NPSH Required

Basic Hydraulics Typical Pump Performance Curve

Brake Horsepower

Basic Hydraulics Typical Pump Performance Curve

Basic Hydraulics Published Pump Performance Curve

Basic Hydraulics Performance Curve Limitations

Idealized pump performance @ Datum ◆ Not every pump unless test requested ◆ HI pump test tolerances ◆ HI piping conditions ◆ Deaerated water not user’s pumpage ◆ Use as a guide to pump performance ◆

Basic Hydraulics

Net Positive Suction Head

Basic Hydraulics Net Positive Suction Head

The Head on the Suction Side of the pump needed to provide adequate flow into the pump operating at a given flow rate. Roughly equal to Barometric Pressure plus or minus Static Head less Friction Losses in the Piping and Vapor Pressure of the liquid.

Basic Hydraulics

Atmospheric Pressure

Basic Hydraulics Atmospheric Pressure

Basic Hydraulics Net Positive Suction Head

Atmospheric Pressure provides the force: 14.7 PSI at Sea Level = 33.9 Ft. of Head 14.7 X 2.31 Ft / PSI = 33.957 Ft. As the Pump Impeller reduces the pressure in the eye atmospheric pushes the water into the pump.

Basic Hydraulics Net Positive Suction Head

Static Lift Condition:

Level below the centerline of the pump. Losses include the Lift, Friction and Vapor Pressure of Liquid. Static Head Condition:

Level above the pump. Static helps, Friction and Vapor Pressure hurts.

Basic Hydraulics Net Positive Suction Head Vapor Pressure

The pressure below which a liquid will change phase from a liquid to a gas. Is influenced by Temperature. Example: Water at 212 Deg. F has a V.P. of 14.679 PSI At 80 Deg. F it has a V.P. of .500 PSI

Basic Hydraulics Net Positive Suction Head Vapor Pressure

To boil water (change phase) you can heat it to 212 Deg. F or reduce the pressure on it to about .5 PSI. The result is the same: The Water Boils… changing phase from a liquid to a gas. That gas is called Water Vapor, if we can see it it’s called steam.

Basic Hydraulics Net Positive Suction Head Vapor Pressure Temp. F

Temp. C

SG

Vapor Pressure

70

21

.999

0.36

160

71

.979

4.71

212

100

.959

14.69

400

204

.860

247.31

Basic Hydraulics Net Positive Suction Head Vapor Pressure

If the pressure drop at the center of the impeller is great enough, the liquid changes phase in the eye of the impeller, a bubble of vapor is created, blinds off the impeller and the pump stops pumping; it is said to have “Lost it’s Prime”. Called “Flow or Column Separation”

Basic Hydraulics Net Positive Suction Head Vapor Pressure

If you shut the pump off the bubble escapes from the center of the impeller, is replaced by liquid and turning the pump back on causes it to pump once again.

Basic Hydraulics Net Positive Suction Head How Is NPSHR Determined On The Test Stand?

Typically based on test methods per Hydraulic Institute Test Standards. Based upon a 3% drop in total head. Note the published NPSHR is after you’ve already lost 3% of the the Discharge Head!

Basic Hydraulics Net Positive Suction Head T o t a l H e a d F e e t

Constant Flow of Total Head

Ex. hgs. 3% = 8 ft.

Basic Hydraulics Net Positive Suction Head NPSHR = (hatm + hgs, 3% - hvp) + Zs + hvs SG Where: hatm = Barometric pressure, ft. hgs, 3% = Suction gauge pressure @ 3% drop in head, ft. hvp = Vapor pressure, ft. Zs = Suction gauge height, ft. Hvs = suction velocity head, ft. SG = Specific gravity

Basic Hydraulics

Cavitation

Basic Hydraulics Cavitation If the NPSH Available in the System drops to or below the NPSH Required by the pump the Pump will Experience Cavitation. It is the continuous formation and collapse of Vapor Bubbles in the Impeller or Casing and causes damage.

Basic Hydraulics Cavitation Pressure at the eye of the Impeller or elsewhere drops to below the Vapor Pressure of the Liquid and Bubbles form. As the bubble moves through the Pump the Pressure increases and the Bubble collapses. It is in the collapse the Damage is Done.

Basic Hydraulics Cavitation

Low Velocity / High Pressure

Inc. Vel. / Pressure Drop

Basic Hydraulics Cavitation

Basic Hydraulics Cavitation

Basic Hydraulics Cavitation Bubble collapse creates intense pressure, up to 10,000 psi and severe shock waves. Pressure and shock near metal surfaces exceed material strength and fatigue metals. Fatigued metal breaks away creating pitted surfaces. Pitted areas become concentration points for further collapse of bubbles.

Basic Hydraulics Cavitation

Basic Hydraulics Cavitation Cavitation can only be avoided by having adequate -

NPSH Margin The difference between NPSH available and NPSH required by the pump. Changes with Flow Rate - High flows require higher NPSHa

Basic Hydraulics Cavitation NPSH Margin “Rule of Thumb” The NPSH Margin should be at least 3 Ft. or 10 % of the NPSHr greater than the Required. Within reason “More is Better” Watch for “High Suction Specific Speed Pumps”

Basic Hydraulics

Speed & Impeller Changes

Basic Hydraulics Speed & Impeller Changes

A Customers Desired Operating Point can be marked on the Curve to Graphically show where it falls in relationship to the Efficiency, Brake Horsepower, and NPSH.

Basic Hydraulics

Basic Hydraulics Speed & Impeller Changes

In the previous Slide the Customers Desired Point of 200 GPM at 120 Ft. of Head falls exactly on the Curve for a 5 - 1/2” Dia. Impeller.

Basic Hydraulics Speed & Impeller Changes

What if the Desired Point were 200 GPM at 140 Ft.? How would we select the correct Speed or Impeller Diameter?

Basic Hydraulics

Basic Hydraulics

“Affinity Laws”

Basic Hydraulics Affinity Laws

Head and Flow are related to the Velocity. Velocity is related to Speed and / or Diameter of Impeller. Therefore, Head and Flow vary as the Speed or the Diameter Varies.

Basic Hydraulics Affinity Laws

The change is very Predictable using the “Affinity Laws”. Q1 = D1 or N1 Q2 D2 N2 H1 = D1 2 or N1 H2 D2 N2

2

Basic Hydraulics Affinity Laws

“Affinity Laws” further state:

BHP1 = D1 BHP2 D2

3

or N1 N2

3

Horsepower goes up at the Cube of the Speed or Diameter change. A 20% increase in speed causes a 70%increase in Horsepower!

Basic Hydraulics Affinity Laws

Example: 3175 @ 880 RPM w/ 13.5” Impeller How much change in Head, Flow and Horsepower with 14.5” Impeller? This is a 7.5 % Increase in Impeller Diameter.

Basic Hydraulics

Affinity Laws

Flow: 1400 = 13.5” Q2 14.5” Q2 = 1 / 13.5 X 1400 14.5 Q2 = 1503 GPM (7.5% Inc.)

Basic Hydraulics Affinity Laws

Head:

40 = H2

13.5 14.5

2

H2 = 1 / 13.5 2 X 40 14.5 H2 = 46 Ft. (15% Inc.)

Basic Hydraulics Affinity Laws

BHP:

20 = Hp 2

13.5 14.5

3

Hp 2 = 1 / 13.5 3 X 20 14.5 Hp 2 = 25 Hp.( 24% Inc!)

Basic Hydraulics

Parallel & Series Operation

Basic Hydraulics Parallel & Series Operation

Multiple Pumps in Operation on a System can be Parallel, Series or Separate Operation

Basic Hydraulics Parallel & Series Operation Series Operation:

One pump discharges into the suction of a second pump. Head is cumulative, flow is constant Slurry Pipelines, Vertical Multi-Stage Turbine Pumps.

Basic Hydraulics Parallel & Series Operation Series Operation:

Normally where one pump at a given speed can’t produce the head required multiple pumps are used. One Pump 125 Ft. of Head Add a second = 250 Ft. a Third = 375 Ft.

Basic Hydraulics Parallel & Series Operation System Curve Two Pumps

300 GPM @ 200 Ft.

One Pump

133 GPM @ 120 Ft.

Basic Hydraulics Parallel & Series Operation Parallel Operation:

More than one pump draws from the same suction source and discharges to the same discharge line. Flow is cumulative, Head varies as the flow increases. Municipal Sewage and Water Supply Applications.

Basic Hydraulics Parallel & Series Operation Parallel Operation:

If the System Curve was perfectly flat the flow would be equally cumulative. Two Pumps would equal twice the flow.

Basic Hydraulics Parallel & Series Operation Real System Curve

One Pump

Two Pumps

Flat System Curve 300 GPM

600 GPM

Basic Hydraulics Parallel & Series Operation Parallel Operation:

Because the Friction Head goes up as the flow, Curve becomes steeper, two pumps do not equal twice the flow.

Basic Hydraulics Parallel & Series Operation System Curve 200’ 3” Pipe 4 Elbows 1 Foot Valve

One Pump

2 Globe Valves

266 GPM @ 175 Ft.

Basic Hydraulics Parallel & Series Operation System Curve

One Pump

295 GPM @ 190 Ft.

Two Pumps