#249 A plant located at an altitude of

A plant located at an altitude of - Mechanical Engineering

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Free Chegg Question

2. A plant located at an altitude of 2.500 ft. (atmospheric pressure 27.5 in Hg) has an open feedwater heater located 10 ft. above the pump centerline. The water in the heater has a temperature of 190°F. If the head loss in the line between the heater and pump is 1 ft., determine the available suction head at the pump.

3. Determine the available suction head of a pump which is taking gas oil at a temperature of 400°F. from a closed tank in which the pressure is 85 lb. per sq.in. gage. The specific gravity of the oil is 0.78 and its vapor pressure is 90 lb. per sq. in. absolute. The loss in the suction pipe is 2 ft. and the pump is located at 12 ft. above the oil level in a plant at sea level.

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Free Chegg Answer

To the honor student
The problem solving involves the concep of net positive suction head (NPSH).
The system NPSH availabe in a proposed installation can be calculated by the formula:

Hsv = Hp + Hz - H_f - Hvp

Where Hsv = NPSH expressed in feet of fluid

Hp = absolute pressure on the surface of the liquid where the pump takes suction expressed in feet of fluid.

Hz = Static elevation of the liquid above the centerline of the pump (on vertical pumps the correction should be made to the entrance eye of the impeller) expressed in feet. if the liqid level is below the pump centerline, Hz is minus.

H_f = Friction and entrance head losses in the suction piping expressed in feet.

Hvp = absolute vapor pressure of fluid at the pumping temperature expressed in feet of fluid.

As the some parameters we have are in pressure so in terms of pressure we were expressing NPSH as below

Hsv = Pa + Ps + (V_s² / 2g) - Hvp

Where

Pa = atmospheric pressure for the elevation of the installation expressed in feet of fluid.

Ps = gage pressure at the suction flange of the pumbp corrected to the pump centerline and expressed in feet of fluid, Ps is minus if it is below atmospheric pressure.

(V_s²/2g) = velocity head at the point of measurement of Ps

Hvp = absolute vapor pressure expressed in feet of fluid.

Question 2

The case was open tank with liquid level above the pump centerline

For this application Hp = Pa

Hz is positive
Calculated available system NPSh
Hsv = Pa + Nz - H_f - Hvp
And the given avaliable data is Atomspheric pressure Pa = 27.5 in Hg
Conversion into feet : Pa 27.5 x 1.133
= 31.16 ft of water
Pa = 31.16 specific gravity of water
= 31.16 / 1
= 31.16 ft

Open feet water heater located at 10 ft. above the pump centerline i.e, Hz = 10 ft
The head loss in the line between the heater and the pump is H_f = 1ft.

The heater tempreature is 190 0F, so we take vapor pressure

substiture all these values in the NPSH equation

Hsv = Pa + Hz - H_f - Hvp
= 31.16 + 10 - 1 - 10.58
=29.58 ft
The suction head avaliable in the pump is = 29.58 ft.

Question 3

The case was closed tank with liquid level above the pump centerline

It should be noted that an installation with a so called closed system as shown above the liquid source from which the pump takes suction exists at its saturation pressure for the prevailling temperature. Thus, vapor pressure of the liquid equals the pressure on the surface of the liquid and the two terms of the basic NPSH equation nullify each other.

For this application
Hp - vapor pressure of fluid = Hvp
Hsv = Hvp + Hz - H_f - Hvp
Hsv = Hz - H_f
Calculated available system NPSH
Hsv = Hz - H_f
Where Hp = Hvp
And the given available data is
The specific gravity of the oil is 0.78
its vapor pressure is = 90 lb
The loss is suction pipe is, H_f = 2 ft.

The pump located above centerline at, Hz = 12 ft.
We know the formula for finding suction head in the case of closed tank at above centerline is
Hsv = Hz - H_f
= 12 - 2
= 10ft.
The available suction head of the pump is = 10ft