Vertical vs. Horizontal Orifice Plate Install 2

[lacajun]

Some guys I am working with have an old Shell Refining Handbook that states orifice runs are best installed in vertical runs. Flowing up for liquids and down for gases and steam.

This is counter to everything I have learned in my 18 years about flow metering with orifice runs. The Chilton/CRC/ISA Measurement Handbook says horizontal runs. From an engineering perspective, it doesn't make sense for accuracy either.

Have any of you heard of this before? Know where this thinking came from? What are your thoughts about installing orifice meters in vertical runs and horizontal runs?

 

[zdas04]

Let me apologize in advance, this is going to be a long post, but you've hit one of my many hot buttons.

The 1960's thinking was that a vertical meter run (down for gas) would be inherently good at separating liquids out and would prevent multi-phase complications in measurement (I suppose they thought the gas would outrun the liquid?). For liquids you wanted to flow up because many field measurement applications are flowing less than a full pipe and the "thinking" was that vertical-up would ensure that the pipe around the plate was full. Neither of these rationalizations stood up to systematic laboratory study.

The arithmetic for converting a P, dP, and T to a volume flow rate starts with Bernoulli's Equation and applies several dozen assumptions. The first is that the change in elevation term can be discarded because "tubes are always installed on the horizontal", and then doesn't require horizontal installation. For gas the density is so small that the dP due to elevation change of just over 2 inches is probably insignificant on any commercial scale, I just hate sloppy specifications.

The current AGA-3 is silent on tube orientation. I was having a discussion with a client a couple of years ago about their vertical tubes in custody transfer gas measurement. I went to some people I know on the committee and asked what the thinking was. The answer I got was that they wanted to require horizontal orientation, but the offshore lobby had kittens about the extra footprint that would be required and the latest version is totally silent on tube orientation so either horizontal or vertical tubes are allowed in the spec (that has the force of law in many circumstances). I finally had to withdraw my exception from the report I was preparing, tuck my tail firmly between my legs, and slink off. I'm still bitter about that experience.

Bottom line is that vertical tubes can be supported by the silence of the code, but they are not a good measurement practice.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[JLSeagull]

Refer to the API Manual of Petroleum Measurement Standards
Chapter 5—Metering, Section 7—Testing Protocol for Differential, Pressure Flow Measurement Devices, FIRST EDITION, FEBRUARY 2003

4.7 METER AND SECONDARY INSTRUMENT ORIENTATION
Meters are normally tested in the horizontal orientation. In general, testing meters in the vertical orientation may be difficult. However, the meter should be tested in the orientation in which it is to be used. For meters installed vertically, differential pressure readings may need to be corrected as defined by the manufacturer for the relative elevation of the pressure taps. Differential pressure transmitters are sensitive to mounting position orientation. To minimize the effects of orientation the transmitter must be zeroed after installation.

 

[Ashereng]

All of my orifice plate meter runs are horizontal.

Having said that, I have the luxury of real estate in my projects.

I would think that a vertical run would be also good, assuming the pipe if filled (liquid) and single phase flow. In some cases, physical space limitation dictate what we do.

I don't know whether a vertical installation would be less accurate than a horizontal one. Maybe someone has comparison data to share?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[dcasto]

In the verticle, good luck with making the correction to the DP transmitter, you will have to write a custom subroutine to compensate for that the EFM's I'm familiar with do not have an option for that.

That said, I'll bet most refinery meters do not meet API measurement standards anyway, so hang it any way you want. They probabily use a dcs to totalize too, yikes!!

 

[JLSeagull]

The accuracy would be an issue for custody transfer or Royalty meters. Normal flows within a unit are not custody transfer meters. If the range is 0-100 inch-water and the head difference is one-inch water then one percent error would exist. However the actual difference at the vena contracta due to head may not be as obvious as it seems.

 

[quark]

Good post David and a star for you. Why do you, still, prefer horizontal installations?

Though I have the luxury of having horizontal spaces like Ashereng(I am always confused when people say things about saving space)and kept the practice of horizontal installations so far, I saw some installations of orifices for custody transfer applications.

Compensation incase of steam is known to me but I am not knowledgeable about why it is required for pure gases that are not too hot. I think dcasto is referring to steam and liquids.

 

[dcasto]

Because we have the ability to calculate and compensate. In gas flow there is a compensation for the density change of the gas across the plate, Y factor. We now must iterate all flow measure as there is no direct solution. The reason is we have integrated the reynolds number adjustment into the coefficient of discharge calculations.

The other reason we use horizontal is if you use flow conditioners, the flow condioner manufacture must prove his unit will comply with the standards in that position, so its to the lab to prove that.

 

[zdas04]

I prefer horizontal tubes because I am truly an old fuddy duddy. I know in my brain that with proper transmitter installation the results you can get from a single-phase vertical tube are within contractually-required accuracy, have good repeatibility, and similar uncertainty as horizontal tubes.

My heart tells me that it is cheating to assume that elevation cancels and then take a step that invalidates that assumption. I've gone through the derivation of the current generation of measurement equations (yes, I'm that sort of geek) and I don't think that they could have been derived without throwing out the elevation term in Bernoulli's Equation (at least I couldn't have done it).

If I ever have a client who demands that I include a vertical meter tube in a design I'll probably put on my 4-inch heals and hot pants and do it for them (yeah, I'm a whore, but I get paid well for it and have NEVER gotten anyone hurt and stations that I design measure gas well).

David

 

[lacajun]

Thank you all for posting.

When I began in instrumentation 18 years ago, the older engineers discouraged installing orifice meter runs in vertical piping because you cannot guarantee full pipe flow for liquids. That was in an existing plant with lots of real estate and lots of room for horizontal runs. Maybe that influenced ideas.

Seems like one mechanical engineer had experienced pressure issues in vertical piping, too, but I forget now. Seems like he preferred horizontal runs from a system pressure drop perspective. But not being a fluid dynamics expert, I won't jump through those hoops. But in my pea brain, I can't see how that would make a difference. But if any of you mechanical types know or disagree, I am all ears.

Plates have holes in the top for gases to pass through in liquid flows and holes in the bottom for liquids to pass through in gas flows. In my pea brain, it seems in vertical piping, those holes are meaningless and everything passes through the orifice. Is my thinking muddled here?

In this application, only a volumetric flow is needed.

 

[dcasto]

If you have extra holes in the plate, there goes your measurement accuracy. However, it has been shown that if you add the area of the weep holes to the area of the orifice, then calculate a new diameter and use the calculated diameter, you do not end up as far off as without that corection.

 

[zdas04]

dcasto,
I've run some of those tests at CEESI and the problem I had was repeatability with my construct diameter. It almost acted like in some conditions the surface tension of the fluids was greater than the dP across the plate and the weep holes didn't contribute to flow (I know this isn't possible with their air rig, but that is the way it acted). Anyway each test was a different error percent.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[dcasto]

I had a plant balance problem and after reading Miller, I followed the book. Over several 1 month periods the plant balance got closer.

 

[lacajun]

If I needed better measurement for mass balances, I would install Micro Motion Coriolis meters. The accuracy and rangeability are hard to beat. We are using those on mass balance streams.

The vent/weep holes should prevent significant buildup of stuff you aren't trying to measure to keep the flow profile undistorted and prevent measurement fluctuations.

I have never had to account for weep/vent holes but it probably would have helped. Orifice runs aren't too accurate to begin with, which may be why most of the instrument engineers I know haven't dealt with it much.

 

[zdas04]

Orifice runs aren't too accurate to begin with

What an amazing statement. There is no less than $100 million PER DAY tht changes hands based on orifice measurement. The standards in AGA specify 0.5% uncertainty. Every other type of measurement is compared to orifice measurement and typically the other forms (including Coriolis meters) have been found wanting.

I just can't beleive that anyone would make a statement like that in a measurement forum.

David

 

[Ashereng]

I have never had to account for weep/vent holes but it probably would have helped. Orifice runs aren't too accurate to begin with, which may be why most of the instrument engineers I know haven't dealt with it much.

Most of instrument guys that I work with are familiar with orifice meters.

I also agree with David that most custody meters (that I see) are orifice meters.

Some reasons for not using a orifice meter may include:
- upstream and downstream requirements
- not custody transfer
- larger turndowns required (typical orifice turndowns that we use is 5:1)
- mass measurement (I agree, if you need mass, a coriolis is a good way to go) since you need density to convert flow rate to mass rate.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[dcasto]

The US ethylene market is 100% orifice based, @ 78,000 tonnes per day and $.6 / lb thats over $100,000,000 per day bought and sold (not including resales), all orifice based measurement. Most plant instrument guys are familiar with meters, they are not the experts, leave measurement to the professionals.

 

[lacajun]

Is this a forum to read only?

I am sorry to have posted here.

 

[quark]

Is this a forum to read only?

No, this is perfectly an interactive forum and your ideas and experience are always welcome.

I agree with you that orifice meters are not too accurate but your statement of instrumentation engineers not preferring orifices is not true.

In my opinion, custody transfer application doesn't require accuracy of the measuring device but only accuracy of measurement. Once the measuring device is calibrated and error is identified, we require only repeatability. As orifice meters have good repeatability, they suit the application.

Ashereng summed it up nicely.

 

[itsmoked]

Quote (lacajun): Is this a forum to read only?

lacajun; your comment makes pretty much ZERO sense to me. My first thought was "has lacajun had a stroke since his last post"?

Perhaps you'd like to enlighten us further about your comment? How are we not meeting your expectations?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com