Attention to: PT. Semen Tonasa Construction Department (Area-4)

- Mr. Zaki / Mr. Agus Sulaeman

PT. Semen Tonasa Engineering Department

- Mr. Alam / Mr. Nanti S Tambun

From: PT. Wiratman

Subject: NPSH Assessment of existing circulating water pump &

Modification proposal of suction piping 552-WP3A & B

Preliminary description As reported by PT. Semen Tonasa staff that both pumps that used for circulating water from

cement mill pond to main recirculation pond are not suitable for operation due to

cavitations occurred.

Assessment Points 1. Is NPSHA > NPSHR? Or what is depth of lower limit switch needs to be positioned to

deactivate the pump, to avoid cavitations occur and broken the pumps?

2. Are any unnecessary suction line design or construction that could results excessive

pressure drop or any leakage possibilities?

NPSH assessment

NPSHA shall be greater rather than NPSHR for suitable operation, the rule of thumb

minimum NPSHA ≡ 1.15 NPSHR. The objective of this assessment is to determine elevation of

the installation of pump and the pressure drop due to pipe friction and pipe accessories still

in pump working range.

Determining Z (maximum level difference between pump axis and reservoir level)

And overall pressure drop are important to avoid cavitations

NPSH assessment flow chart

To get NPSHR pump

data

Type of pump 125x100 FSR (end suction volute pump)

NPSHR: 4 meter [@1500 rpm]

Head and debit: 11 m, 150 cum/h [55.5KW @1500 rpm]

To get Fluid

properties

Fluid properties – water at 20°C and 100 kPa (absolute)

Density: 998 kg/cum

Viscosity: 1.0020 cp

Vapour pressure: 2.4 kPa (absolute)

Determining

additional losses

Additional losses ∑(F):

Minor losses (foot valve include strainer) and Major losses

(suction pipe) ≈ 1m

Determining

Maximum level

difference between

pump and reservoir

fluid surface

Rule: NPSHA > NPSHR

Determining minimum NPSHA to avoid cavitations is:

Minimum NPSHA ≡ 1.15 NPSHR (condition might vary from 1.10

up to 1.15)

Minimum NPSHA = 1.15 * 4 m = 4.60 m

NPSHA (m)equation = PO/ρ.g + Z - ∑(F) – Pvap/ρ.g

PO: atmospheric pressure [Pa.abs]

ρ: Fluid density [kg/cum]

Z: Max level difference between pump & reservoir surface [m]

g: Gravitational acceleration [m/s^2]

∑(F): Additional losses [m]

Pvap: Vapour pressure of water at 100kPa & 20OC [Pa.abs]

4.60 m = (100,000/(998*9.81)) + Z – (1) – (2,400/(998*9.81))

Z = - 4.369 m ≈ 4.3 m

∴ Maximum pump elevation installed above reservoir surface

level is 4.3 m, so at this point level limit switch recommend to

install for avoiding pump broken due to cavitations.

Suction line assessment

Existing installation, as per inspected on Thursday June 21, 2012

� Are any unnecessary accessories installed that affect to the excessive pressure drop?

YES

1. Butterfly valve can be eliminated; flow control shall be placed at discharge of pump.

Indeed head loss for butterfly valve can greatly reduces NPSHA

2. Strainer should be replaced with foot valve which has built in strainer inside. Strainer

reposition can reduce pressure drop, and foot valve also as check valve that keep water

not felt down to the reservoir.

� Are any possibilities of leakage? YES

3. Due to pump configuration works is 2x100%, so one pump is working and another is

standby. So leakage could be from standby pump and water from reservoir would not to

be lifted. So individual suction line shall be provided.

Modification proposal for suction lines

1

2

3

Modifications involve:

1. Providing individual suction line for both pumps.

2. Eliminating butterfly valve at suction line

3. Replacing standard strainer with foot valve (built in strainer), foot valve function as

check valve.

Additional accessories that need to buy:

No Accessories name Spec Qty Remark

1 Foot valve with

strainer

Size: 6”

Connection: Flanged class 125# (min)

Application: Water

Strainer size: 80 Mesh,

Strainer material: Stainless steel A 316

Material body: A216

2pcs Please find in

attachment

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Best Regards,

PT. Wiratman