Pressure drop due to contractions or expansions.

[dbecker]

Hello,

I have two questions,

1. When calculating the pressure drop due to an abrupt contraction, do I use the minor diameter or the major diameter with equation 3-14 of Cranes (page 3-4)? It calls for a diameter, and I don't know which to use.
Same question pertains to an expansion.

2. I noticed there are a few equations out there on calculating losses due to contractions.

One equation is, K=0.5(1-beta^2) for 180 degree abrupt contractions as seen in Crane's page A-26 Formula 2.

The other is K=0.5(1-beta^2)^(3/4). I got this from the web.

Why is one raised to the 3/4 power and the other is not, this deviation is confusing and I don't see a case basis where one is used and the other is not.

Please explain.

Thank you!

 

[rneill]

1. It does mention in the Text of Crane that if you use K1 values in Eqn 3-14 then velocity is based on smaller pipe if you use K2 values then velocity is based on larger pipe. See the note following Eqn 3-18.1 on page 3-4 (assuming you have the same version I have).

2. My reference texts (although a bit dated) only show the equation in Crane or slight variations of it (e.g., different coefficients such as 0.42 rather than 0.5). None have an equation with a different power although I did a quick search and found the same as you ?

 

[dbecker]

Thank you rneill.

I read that note but it only refers to equations 3-17, 3-17.1 and 3-18, i did not know it also referred to equations 3-14. But thank you for clarifying that anyhow.

If I am correct, as beta approaches zero (D1~0) K will approach 0.5.