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Water piping design

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Fundamentals of water circuits

• Open circuits

P HeatSource

P

Load

• Closed circuits

Source

ET

Thermal tank Cooling tower …

Expansion tank

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Learning, our wayComparison Open versus Closed

• Higher output of pump motor (higher actual head)

• Proper water treatment is required (avoid corrosion)

• Sufficient air purging

• Requires a properly installed expansion tank.

Open circuit Closed circuit

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Learning, our wayWater piping systems.

• Supply of water according to the needs of the user of the required places.

• Head and friction losses should be minimal

• Water velocity should be properly managed to avoid streaming noise, pipe vibration, pipe expansion, … .

• Water quality management

• Arrangements for easy service and maintenance.

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Learning, our wayWater piping systems - classifications

• Return water method

• Water pipe number method

• Flow control method

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• Direct return

• Reverse return

Water piping systems – Return water method

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Learning, our wayWater piping systems – Direct return method

Recommended when the units have different pressure drops or require balancing valves.

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Learning, our wayWater piping systems – Reverse return method

Recommended when the units have the same pressure drops and for most closed piping systems.

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Learning, our wayWater piping systems – Piping number method

• Single pipe method

• Two-pipe method

• Three-pipe method

• Four-pipe method

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Learning, our wayWater piping systems – Single pipe method

Used in small scale hot water heating applications

Flow control is difficult Low installation cost

Disadvantage: Advantage:

Chilled or Hot water Orifice

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Learning, our wayWater piping systems – Two pipe method

Chilled or Hot water

Most commonly used system.

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Learning, our wayWater piping systems – Three pipe method

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Learning, our wayWater piping systems – Four pipe method

COLD COIL

ReturnChilledWater

Chilled WaterSupply

Hot WaterSupply

HOT COIL

ReturnHotWater

Unit Thermostat

Unit Thermostat

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Learning, our wayWater piping systems – Flow control method

• Constant flow method

• Variable flow method

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Darcy equation:

ΔP = f l ρ * v

2

d 2* *

∆P = pressure lossρ = fluid densityf = friction factor (Pa / m)l = pipe length (m)v = fluid velocity (m/s)d = internal pipe diameter (m)

Water piping design – friction losses

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Learning, our wayWater piping design – friction losses

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Learning, our wayWater piping design – water velocity

The recommended water velocity is depending on: Pipe diameter Effects of corrosion

The higher the water velocity: The higher the noise level The higher the effects of erosion

Pipe diameter (mm) Velocity range (m/s)-----------------------------------------------------------------------------125 2.1~2.750 ~ 100 1.2~2.1Around 25 0.6~1.2

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Learning, our wayWater piping design – expansion tank

Purpose:

maintain the system pressure by allowing the water to expand when the water temperature increases.

Provide a method to add water to the system To release air contained in the water system.

An expansion tank is required in a closed system.

The expansion tank can be of the open or closed type.

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Learning, our wayWater piping design – expansion tank

Location: suction side of the pump, above the highest point in the system

Sizing:1. Calculate the water volume in the piping (tables)2. Calculate the water volume in the heat exchangers

(engineering data books of the manufacturers)3. Determine the specific volume both for the lowest and

highest working temperature and calculate the difference.4. Calculate the required volume of the expansion tank

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Water piping design – closed expansion tank

Vt = 2 * Vs[(v2 / v1 -1) – 3 * α *Δt]

[(v2 / v1) -1] – 3 * α *Δt

Water piping design – open expansion tank

Vt = Vs * ( Pa / P1 ) – (Pa / P2)

Vt = volume of the expansion tank (m³)Vs = volume of the water in the system (m³)t1 = lower temperature (°C)t2 = higher temperature (°C)pa = atmospheric pressure (kPa)p1 =pressure at lower temperature (kPa)p2 = pressure at lower temperature (kPa)

v1 = specific volume at t1 (m³/kg)v2 = specific volume at t2 (m³/kg)α = linear coefficient of thermal exp. = 11.7 * 10-6 m / (m*K) for steel = 17.1 * 10-6 m / (m*K) for copperΔt = (t2 – t1) (K)

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Learning, our wayWater piping design – pump types

Most frequently used pumps are the centrifugal pumps. More and more variable flow (inverter) pumps are used

which offer the following advantages over the constant flow pumps:

Application of load diversity to the design allows approximately 70 to 80% of the peak building load.

No margin required for balancing of the circuits, saving possibly 10% flow.

Cost saving in material and labour for the main and branch piping system

Reduction in commissioning time.

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impeller

tongue(cut off plate)

spiral shaped water passage

pump body

rotation

inlet

Water piping design – pump types

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Learning, our wayWater piping design – pump performance

Pump performance can be given in terms of:

discharge capacity= required flow rate head shaft power efficiency

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Learning, our wayWater piping design – pump performance

Pump performance can be given in terms of:

discharge capacity head

0

1

2

3

4

5

6

7

8

9

0 2 4 6 8 10 12

Capacity

To

tal h

ead

Tot

al h

ead

= pressure produced by the pump

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Learning, our wayWater piping design – pump performance

Pump performance can be given in terms of:

discharge capacity head shaft power

0

0,5

1

1,5

2

2,5

3

3,5

4

0 2 4 6 8 10 12

Capacity

Sh

aft

po

wer

= required power of the pump is roughly proportional to the delivered capacity.

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Learning, our wayWater piping design – pump performance

Pump performance can be given in terms of:

discharge capacity head shaft power efficiency = ratio between the delivered work and the

shaft power.

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10

Capacity

Eff

icie

ncy

(%

)

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Learning, our wayWater piping design – pump performance chart

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Learning, our wayWater piping design – pump selection

Where is the operating point?

What is the system resistance curve?

What design flow rate is required?

What is the pressure drop?

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Learning, our wayWater piping design – pump selection

System resistance curve

Total Head

Capacity

Total head

Operatingpoint

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Learning, our wayWater piping design – pump selection

System resistance

curve

Total Head

Capacity

Total head

Operatingpoint

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Learning, our wayWater piping design – pump selection example.

H = Ha + Hf + Ht + Hk

H = Total friction loss (m H2O)

Ha = Actual head (m H2O)

Hf = Friction loss in straight lines

Ht = Partial friction loss

Hk = Internal friction loss

Ha = 0 for a closed system

Hf can be obtained from the friction loss diagram

Determine the equivalent length of the valves, strainers, …Can be obtained from the manuf. tech. data.

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Learning, our wayWater piping design – pump selection example.

Given information: WFR = 190 l/min friction loss at the foot

gate = friction loss of the check valve

water temperature: 30 °C

Foot valve

1 m 10 m 2 m

7 m

2 m

6 m

4 m

2 m

Check valve

Gate valve

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Learning, our wayWater piping design – pump selection example.

Pipe diameter (mm) Velocity range (m/s)-----------------------------------------------------------------------------Main lines of pump discharge 2.4~3.6Main lines of pump section 1.2~2.1

The water velocity at the suction side

Based on the WFR of 190 l/min and the limits of the water velocity, we have to select a 2B steel pipe.

The intersection between the WFR and the 2B pipe gives us the following data:

water velocity: 1.4 m/s friction loss: 0.060 mmH2O / m

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Learning, our wayWater piping design – pump selection example.

Actual head: suction: 4 mdischarge: 15 m 19 m H2O

Friction losses in straight linesuction: 2+4+1 = 7m * 0.060 mH2O = 0.42 m H2Odischarge: 7+10+6+2= 25m * 0.060 mH2O = 1.50 m H2O

Partial friction loss Equivalent length Qty90° elbow 1.6 mH20 * 4 = 6.40 * 0.060 mH2O= 0.38 mH2OCheck valve 4.1 mH2O * 1 = 4.10 * 0.060 mH2O= 0.25 mH2OFoot valve 4.1 mH2O * 1 = 4.10 * 0.060 mH2O= 0.25 mH2OSluice valve 0.37 mH2O * 1 = 0.37 * 0.060 mH2O= 0.02 mH2O Overall head loss: 21.82 mH2O

Safety factor 1.1

H = 24 mH2O

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Learning, our wayWater piping design – pump selection example.

Pump selection (chart method)

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Thank you for your attention