1 reply to this topic

[deltaChe]

Dear Sir/Madam

I have two traival question need to bother:
Q1.I always misuse the wrong temperature and pressure in
the wrong class pipe. For example, operating pressure 23kg/cm2g
OT=208C, DP=28kg/cm2g DT=300C can't be fitted in the carbon steel
piping.
I wonder that anyone can kindly share any easy-to-read graph for any materail's
PIPING FLANGE RATINGS in different temperature and pressure condition.
Q2. How to judge the pump piping dessign pressure guide rule(attachement).
Suction piping of parallel pumps (downstream of block valve) must be good for the discharge side maximum
operating pressure when valve is closed
Equipment Downstream of Centrifugal Pumps:
Piping - design pressure for piping must be consistent with the design pressure for vessels and
equipment to which it is attached.
How to define the design pressure between pump casing and downstream control valve?
The rule write : DP = Max Suction Pressure + Pump Shutoff P, but I always used shut off pressure of pump
for design pressure of downstream pipe before.
And, the following confuse me more:
From Downstream Control Valve to Last Block Valve:
1.If closing valve can Directly result in max suction pressure:
DP = Max Suction Pressure + Pump Shut-in P
2.If closing valve does not Directly result in max suction
Pressure:
a. DP = Normal Pump Suction Press + Pump Shut-in P
b.DP = Max Pump Suction Press + Normal Pump P
Please help me out how to explain this rule more detail.
Thank you very much.

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•  Line List condition.png   45.61KB   44 downloads

[kkala]

steven7788, on 15 September 2011 - 11:38 AM, asked following queries.

Q1.I always misuse the wrong temperature and pressure in the wrong class pipe. For example, operating pressure 23kg/cm2g OT=208C, DP=28kg/cm2g DT=300C can't be fitted in the carbon steel piping.
I wonder that anyone can kindly share any easy-to-read graph for any materail's
PIPING FLANGE RATINGS in different temperature and pressure condition.
A1. For piping class, design pressure & temperature should be considered, neglecting the operating conditions. In your example design conditions cannot be satisfied by class of 300 Lb (even though operating conditions can), so higher than 300 Lb class should be selected. Attached "lineclas.xls" indicates the difference.
Graphs would indeed be helpful, but I have not found. They could be elaborated for several classes of same material.
Note (α). In case that class limit is marginally lower than what design conditions indicate, these are sometimes modified to match. I have seen this in steam pipes (around 40 Barg, 410 oC operating). Better to avoid this risk and let Piping Dept deal with these marginal matters.
Note (β). Class line in the diagram of "lineclas.xls" assumes interpolation betwen temperatures; few codes dictate flat values of acceptable pressure between two temperature limits (to my information), or max acceptable pressure corresponding to a rather high temperature. Be aware of such code "peculiarities" before selecting classes.
Note (γ). Usually class selection is by Piping or Project Dept, not by Process.
Q2. How to judge the pump piping dessign pressure guide rule(attachement).
Suction piping of parallel pumps (downstream of block valve) must be good for the discharge side maximum operating pressure when valve is closed:
Comment:This pressure is pump shutoff pressure. Assume two pumps operating in parallel (one standby). Discharge flow may pass into the suction of idle pump, if not isolated by error. This safety requirement has been generalized. See http://www.cheresour...h__1#entry38353. More on searching "suction design pressure" in Cheresources forums.
Equipment Downstream of Centrifugal Pumps:
Piping - design pressure for piping must be consistent with the design pressure for vessels and
equipment to which it is attached.
How to define the design pressure between pump casing and downstream control valve?
The rule write : DP = Max Suction Pressure + Pump Shutoff P, but I always used shut off pressure of pump
for design pressure of downstream pipe before.
Comment: The rule is DP=max suction pressure + pump shutoff ΔP (not Pressure), actually giving max shutoff pressure of the pump. In your sketch max suction pressure would be the design pressure of vessel A (equipment).
And, the following confuses me more:
From Downstream Control Valve to Last Block Valve:
1.If closing valve can Directly result in max suction pressure:
DP = Max Suction Pressure + Pump Shut-in P
2.If closing valve does not Directly result in max suction
Pressure:
a. DP = Normal Pump Suction Press + Pump Shut-in P
b.DP = Max Pump Suction Press + Normal Pump ΔP
Please help me out how to explain this rule more detail.
Comment: When valve is closed, pump discharge develops shutoff pressure P = shut-in (i.e. shutoff) ΔP +suction pressure. P = DP in this case. Suction pressure is higher than suction operating pressure just before closure, due to no friction. Suction pressure at that moment can be:
(1) Maximum, e.g. when a PSV is open on vessel A; if A is an atmospheric tank, when liquid is at max level (static pressure).
(α) Normal, e.g. at normal operating pressure of vessel A; or at normal liquid level of tank A; but normal suction pressure takes frictional ΔP into account.
- Max suction press+normal pump ΔP (b above) evidently does not concern case of valve closure, but operation. This rule could be met in some old design practices as an alternative to (α), you had to chose the most conservative among the two.
But applicable today is only (1) as most conservative and simple: DP=max suction pressure + pump ΔP, where max suction pressure = vessel A design pressure (as above). See last posts in http://www.cheresour...h__1#entry35584.
-Looking at the sketch, I believe there should not be class brake just downstream of the control valve. Asume it remains open, while following isolation valve gets closed. In this case piping between the two valve shall face DP of the discharge pipe.
Note 1. Concerning piping DP today, an extra margin is sometimes (not always) added to max pump shutoff
pressure for safety against uncertinties (e.g. water hammer can seriously increase discharge P).
Note 2. Pump casing design pressure should be at least equal to discharge pipe DP, for API 610 pumps (used in refineries) it is usually higher (http://www.eng-tips.....cfm?qid=170147.
Above is hopefully helpfull. "Friction" with application will help confusion to settle down. I have not found books explaining design pressure / temperature matters in depth, apart from design practices of companies.

Attached Files

•  lineclas.xls   16KB   40 downloads

Edited by kkala, 16 September 2011 - 04:11 AM.