MERM Companion Chapter 17 Prob. #30

[Vel2018]

To whoever have this book, can you also please check the solution in this problem? I think the calculation of P1 suction is incorrect.

Problem says suction "gauge" reads 6inHg below atmospheric. 

I translate this as 2.95psi below 14.7psi "gauge" = 11.75psig NOT absolute yet. So the answer to this problem is between option A & B. 

Please help!

 

[MikeGlass1969]

Gauge Pressure would be -2.95 psi. 

Absolute Pressure would be 14.7 - 2.95 = 11.75 psia

You can just use gauge pressures for P1 or P2...  Or you can use absolute pressures for P1 or P2.  Just don't mix gauge and absolute.

 

[Vel2018]

Gauge Pressure would be -2.95 psi. 

Absolute Pressure would be 14.7 - 2.95 = 11.75 psia

You can just use gauge pressures for P1 or P2...  Or you can use absolute pressures for P1 or P2.  Just don't mix gauge and absolute.

I disagree, because it says gauge pressure reads (6inHG=2.95psi) below atmospheric. So the gauge pressure is reading 2.95psi below 14.7. So still that pressure is gauge pressure.

14.7-2.95 = 11.75psi this is what the gauge reads. So in the bernoulli equation if he wants to use the gauge pressure on point one, point 2 should be gauge pressure as well. 

But in the solution, on the discharge side, it added 14.7psi.

 

[MikeGlass1969]

What is the gauge pressure of 14.7 psia (atmospheric condition @ sea level)?  0 psig...

Pabsolute = Patmosphere + Pgauge

 

[MikeGlass1969]

The solution worked the problem in absolute pressures.  You do not need to work the problem in absolute.  You are messing up the gauge pressure, though.

 

[Vel2018]

What is the gauge pressure of 14.7 psia (atmospheric condition @ sea level)?  0 psig...

Pabsolute = Patmosphere + Pgauge

Exactly! Did it say atmospheric gauge? 

This is all about wordings in the problem. If you don't say gauge, what is the assumption? Its in absolute right? 

 

[MikeGlass1969]

The problem states...   

"The inlet suction gauge indicates 6 in (150mm) of mercury below atmospheric."   

If you remove the pressure gauge, what does it read in open air?  It reads 0 psig.  Gauges only read the pressures above or below atmospheric pressure.  If the pressure is below atmospheric pressure it is in a vacuum.  Yet, with your assumptions it's at a positive gauge pressure.  

 

[Vel2018]

The problem states...   

"The inlet suction gauge indicates 6 in (150mm) of mercury below atmospheric."   

If you remove the pressure gauge, what does it read in open air?  It reads 0 psig.  Gauges only read the pressures above or below atmospheric pressure.  If the pressure is below atmospheric pressure it is in a vacuum.  Yet, with your assumptions it's at a positive gauge pressure.  

Yes got it now, since its talking about gauge pressure, so atmospheric should be in gauge too which is 0. I missed that! Damn silly mistakes! 

Thought it was tricking me to get the real reading of pressure gauge but it turns out it was just another term used for pressure in vaccum. Thanks!