Nozzle rating

[wookiyo]

Hi all~

Why one or two stages pumps usually have a same rating in a suction and discharge nozzle?

Could you give me any advice what is the effect in a pump or piping system in case of using different nozzle rating?

Thanks in advance.

 

[BigInch]

I have found it is usually a little better to have the same rating at suction and discharge, but it is often not necessary. The advantage is highly system dependent. To give a few examples,

When there is no flow in the system with a high inlet pressure at the inlet of a "suction" line leading to a pump, head loss in the suction line can be 0 resulting in the development of high pressures at the pump inlet.

When pumps have a recycle line from discharge directly back to suction, in certain conditions almost full discharge pressure can be seen at the pump inlet.

In a long pipeline with intermediate booster stations, if flow is shut down by closing a valve at the end of the pipeline and, for some reason the pump stations do not shut down or experience a delay in the shut-down sequence, high pressure will develop down the entire pipeline at both pump inlet and outlets as flows go to zero and pumps start discharging at their shut-down heads.

Another case would be when a booster station has a very high static pressure to meet before flow can start. With inlet pressure ratings = discharge pressure ratings, you could pump up the suction line to the booster pump as high as possible and minimize the differential pressure the booster is seeing, before starting the booster. The booster will take much less time to add enough differential head to the system to open the discharge check and get flow moving into the high static pressure segment downstream. This can save a lot of heat that would be generated by the booster, if the booster had to run a long time to get up speed <and head> while it could only be on recycle flow.

Its not necessary to have the same suction and discharge ratings, but when you do, it can save a lot of start-up and operating headaches trying to solve those little things that the engineers did not think about when they just designed the system to flow at the "design flowrate".

To find out when you need equal suction and discharge ratings, the system need to be checked for static (no flow) conditions with pumps operating at shut-off head. A transient analysis of start-up and shut-down helps a lot in making the decision as well.


Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Artisi]

It maybe as simple as "why have different flanges ratings on each side of the pump if the flanges are the same diameter?" - just keep everything simple.



Naresuan University
Phitsanulok
Thailand

 

[BigInch]

If they are lower pressure pumps, suction and discharge less than ANSI#150, for which there must be millions, they're the same anyway. Even if they are very high pressure discharge pumps, most all of them take from relatively low pressure sources. Its really not necessary in all cases. Just in some unusual situations where suction pressures, for some reason, could go very high. It seems a lot of those go unnoticed until too late.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[NozzleTwister]

I came on to this thread late...

API 610, 10th edition, Para. 5.4.2.1 states that one- and two-stage pumps shall have suction and discharge flanges of equal rating.

A question I have is, if they were different, what pressure would you hydro the casing at?

Most of my experience is in refineries where we always have an operating pump and spare in parallel. If the spare pump suction valve gets closed and the spare pump discharge valve remains open, then the spare pump and suction line back to the block valve can see the discharge pressure.

In recent years it's been prevalent that the suction block valve and the short piece of piping between that valve and the pump suction flange to be the same Piping Specification and flange class as the pump discharge piping.


NozzleTwister
Houston, Texas

 

[BigInch]

Noz,

Appreciate that, maybe the one I mentioned above was an older pump. Suction was 300# discharge was 900#. Hopefully they hydrotested at the appropriate high pressure. Don't know what test pressure was. And (also hopefully) there was a lot of fat in the low press side flange, with an XXS neck!

Well, just beware, they're out there.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[NozzleTwister]

BigInch,

I think there was a lot of fat in pumps back in the old days. When calculations were done on slide rules, material was cheap and engineering was expensive .... when in doubt, make it stout.

Now with FEA available at your finger tips and materials expensive, to provide a little extra material that's not needed to satisfy the design or the codes is throwing money out the window.

Out of curiosity, I checked an old API 4th edition (1965) that we have laying around here. It stated that the pressure casing shall be designed for the full discharge pressure with 1/8" C.A. and stated "All pump casings preferably shall have suction flanges designed for the same pressure as discharge flanges to permit testing under full test pressure."

Conclusion: Same flange class was not a requirement until a more recent edition.


NozzleTwister
Houston, Texas

 

[BigInch]

There's that word again, "preferably".

You wouldn't just happen to know which edition it came in do you?

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[NozzleTwister]

No, sure don't. It wouldn't be hard to research if you have copies of all the editions.

NozzleTwister
Houston, Texas

 

[BigInch]

No No. I don't. I'm not the kind of guy to ask someone else to do my work (not to say I don't delegate ). If I did, I'd have looked it up and managed to mentioned it already (just as a matter of obvious interest to us both at this point). OK sorry to bother you.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[NozzleTwister]

It's really not of interest to me which edition the suction flange rating became mandatory. I posted on this thread because I thought I could contribute by sharing my experiences.

I don't have ready access to complete copies of all of the API 610 editions. I have the 4th because someone gave me one and I have access to the current edition. I'm a Pipe Stress Engineer and not a Pump Engineer so in an attempt to minimize bookshelf requirements, I have only kept copies of the pages that specifically address pump nozzle loading, which I have kept from each edition since I've been in the business.

Unfortunately, I've thrown away a lot of old codes over the years. I'm still kicking myself in the butt for tossing that 1955 piping code. Let me give some advice to you younger guys, never throw any old Codes or Standards away (at least in your discipline); even if you haven't looked at it for 15 years, you never know when you'll need it on a future revamp. The older the code, the slimmer the chances of finding that information. Recently I've been involved in some refinery revamps of 50-60 year old units. I know how hard it is to find out what the allowable stresses were in 1952 for a material they quit making in the early '60s. It's a lesson learned the hard way when you can't find information form an old code that you had but threw away 10 years ago.

Sorry to get off tangent here, but just wanted to pass on that, just as those old geezers around you are an irreplaceable wealth of knowledge and experience, those old codes are an irreplaceable wealth of information.


NozzleTwister
Houston, Texas