WR PM topics-PE exam

[cantaloup]

Pressure conduit problem

A fire hydrant was installed at one end of a 480 ft long, 6-in cast iron water line. The other end of this water line connected to a 12-in water main. You are asked to calculate the pressure at the fire hydrant if the fire flow is 1000 gpm and the water pressure at the connection to the 12-in main is 50 psi. The ground elevation of the connection of the 6-in water line to the 12- main is 76 ft and the ground elevation of the fire hydrant location is 73 ft. The roughness coefficient for cast iron pipe is 120. Minor loss is negligible.

What is the nearest value of pressure in psi at the fire hydrant?

A/ 27

B/ 30

C/ 33

D/ 36

 

[ktulu]

According to my calcs and assumptions, I came up with about 30 psi. Yes/No???

 

[cantaloup]

Pressure conduit problem
A fire hydrant was installed at one end of a 480 ft long, 6-in cast iron water line. The other end of this water line connected to a 12-in water main. You are asked to calculate the pressure at the fire hydrant if the fire flow is 1000 gpm and the water pressure at the connection to the 12-in main is 50 psi. The ground elevation of the connection of the 6-in water line to the 12- main is 76 ft and the ground elevation of the fire hydrant location is 73 ft. The roughness coefficient for cast iron pipe is 120. Minor loss is negligible.

What is the nearest value of pressure in psi at the fire hydrant?

A/ 27

B/ 30

C/ 33

D/ 36

Solution: Write Bernouilli equation for 2 points: at the 12”X6” tee and at the point inside the hydrant (pressure is given in psi so it has to multiply by 144 to get the pressure terms in ft)

P1*144/gamma + Z1 + V^2/2g = p2*144/gamma + Z2 + V^2/2g + Hloss

(V is unchanged since pipe diameter is unchanged)

Velocity head terms cancelled out.

50*144/62.4 + 76’ = p2*144/64 + 73’ + Hloss (1)

Now calculate Hloss H by HP33s equation solver (or by hand): Q=0.281*C*D^2.63*(H/L)^0.54

Solve for H with Q=1000; C = 120; L=480’; D=6”

Hloss H = 41’

Plug this number in eq. (1) above and solve for p2

191.4 = 2.31*p2 + 73 + 41

p2 = 33.5 psi

C is correct

Note: you can use EPANET water distribution model (download free from EPA website) to verify the result.

 

[Guest]

Note: you can use EPANET water distribution model (download free from EPA website) to verify the result.

Wow .. they still have that program around ??!!! I used the first edition in my Potable Drinking Water Design class :true:

Damn .. I am just old :mellow:

JR