EFFECTS OF WET GAS FLOW ON GAS ORIFICE PLATE METERS Josh Kinney & Richard Steven Colorado Engineering Experiment Station, Inc. 54043 WCR 37 Nunn, Colorado 80648
ifice plate meters are one of the most widely used technologies in industry for gas flow metering. This is due to their relative simplicity, the extensive publicly available data sets that led to several orifice plate meter standards [1, 2, 3, 4] and the fact that they are a relatively inexpensive method of gas metering. However, it is common in industry for gas meters to be installed in applications where the flows are actually wet gas flows, i.e. flows where there is some liquid entrainment in a predominantly gas flow. This is usually done out of economic necessity or due to the fact that the system designers were not aware at the systems conceptual design stage that the gas flow would have entrained liquid. Therefore, with the orifice plate meter being such a popular gas flow meter it is by default possibly the most common wet gas flow meter. The affect of wet gas flow on an orifice plate meter configured for gas flow service is complicated. There are on going research programs worldwide aimed at improving the understanding of the reaction of the differential pressure meter family (of which the orifice plate meter is a member) to wet gas flow. Whereas much of this research is published in recent conference papers it is very technical and is not always immediately relevant to the technician in the field using the orifice plate meter with wet gas how it can be practically applied. This paper attempts to review the current scientific knowledge from a practical users stand point. What is Wet Gas? A practical and universally accepted definition for the term “wet gas” flow has proved illusive over the last decade and despite various suggestions no standards board has released a technical report or standard that has a wet gas flow definition universally agreed upon. In fact, many in the industry now feel there is no need for a precise definition and the term should remain an all encompassing general term while the details of any new research development should simply state the scope in which that product or research is applicable. Hence, for the purpose of this paper the definition of wet gas flow is simply said to be “a two-phase flow of gas and liquid where the predominant phase is that of the gas phase”.
All the same, it is necessary to quantify the relative amount of liquid in a gas flow if we are to discuss the orifice meters wet gas flow performance. This can, and is, done in many different ways. However, for this paper, only two methods are of importance here. These are the straight forward gas to liquid mass flow rate ratio and unfortunately a rather abstract term commonly called the “Lockhart-Martinelli parameter”. The gas and liquid mass flow rates are denoted by m g and ml respectively. The liquid to gas mass flow rate ratio (sometimes simply called the “mass ratio”) is denoted by:
Fortunately, it is not necessary for a technician using an orifice meter with wet gas to have to understand the precise scientific meaning of the Lockhart Martinelli parameter in order to apply the results of the research which utilized this parameter. This term is typically denoted by “ X LM ” and it is calculated by the Equation 1:
X LM =
ρl are the gas and liquid densities of the
wet gas flow. Note that for a measured pressure and temperature and known gas and liquid properties the gas and liquid densities of a wet gas flow should be calculated from fluid property tables or software and this information is therefore known independently of any flow meter. Therefore, for a set pressure and temperature the Lockhart Martinelli parameter is directly proportional to the “mass ratio”. The Lockhart Martinelli parameter can simply be thought of in simple terms as a measure of the “wetness” of the flow. For a set pressure and temperature, the higher the Lockhart Martinelli parameter the wetter the flow (i.e. the more liquid mass flow relative to a unit mass of gas flow).
What Does Wet Gas Flow Look Like? The way that a liquid phase is dispersed in a gas pipe flow is called either the “flow regime” or “flow pattern”. Figure 1 shows a famous generic sketch of typical wet gas flow patterns in pipes. Figure 2 shows a famous generic sketch of typical vertical up wet gas flow patterns in pipes. Vertical down flow patterns, where gravity and flow do not appose each other, are usually considered to be mist flows although little research is published on the matter. Information on inclined pipe flow patterns is very rare in the literature.
over the top of the phase interface. This is a common flow pattern and orifice meter wet gas performance research has been conducted in this flow pattern. However, under different conditions, e.g. faster flow rates and higher pressures, for a set mass ratio this stratified flow has waves at this interface. This is sometimes called “wavy flow” or “wavy stratified flow”. As conditions continue to change for a set mass ratio (e.g. increasing pressure and flow rates) these waves can become large compared to the diameter of the pipe. This flow is sometimes called “semislug flow”. If there is a high enough mass ratio the waves can be large enough to cover the cross sectional area of the pipe and this is called “slug flow”. Semi slug flow and slug flow are not stable flow patterns and therefore these conditions are not the best conditions for flow metering. Finally, if for any set mass ratio, the pressure and gas flow rate values are large enough the gas will flow through the center of the pipe with entrained liquid droplets with a liquid ring flowing on the periphery of the pipe. As the pressure and gas flow rate continue to increase to extreme values the liquid ring will diminish and then disappear as all the liquid is entrained in continually smaller liquid droplets. In reality it is not possible to know the details of the annular ring thickness and average droplet size so these flow patterns are collectively called “annular mist flow”, “annular dispersed flow”, “dispersed flow” or “mist flow”. Vertical up wet gas flow has less flow patterns than horizontal wet gas flow. If there is not a high enough gas dynamic pressure (i.e. the combination of pressure and gas flow rate) there is not enough energy in the gas flow to cleanly drive the liquid phase up the pipe against gravity in a semi-steady / steady way. The liquid then tends to periodically fall back on itself. This occurrence is commonly called “churn flow” due to the violent mixing of the phases. Churn flow is highly unstable and it is strongly advised that metering with any meter is not attempted in this flow pattern. However, if the flow does have a high enough gas dynamic pressure the flow pattern will be a semi steady or steady annular mist flow (or the equivalent terms listed above). Again, as the pressure and gas flow rate continue to increase for a set mass ratio the liquid annular ring will diminish in depth and liquid entrainment in the gas core increases with the average droplet diameter reducing until all the liquid is entrained in droplets. Note, that at very high gas dynamic pressures (i.e. high pressures and gas flow rates for set mass ratios) for either horizontal or vertical up wet gas flows the mist flow could have such small average droplet size that the liquid could be called atomized and the resulting flow could be called a “pseudo-single phase flow” or a “homogenized flow”. That is, the wet gas flow starts to behave like a single phase fluid as the two phases are so well mixed. This is a common assumption amongst some in the industry but a
Figure 1. Horizontal Wet Gas Flow Pattern Maps.
Figure 2. Vertical Up Wet Gas Flow Pattern Maps. The majority of the wet gas flow pattern map literature is for horizontal flow and vertical up flow. Even here, there is not a great deal of literature available. Most knowledge comes from practical experiment and the mathematical models that exist are still in their infancy, do not predict the flow patterns well across various parameter changes (e.g. pipe diameter, line pressure, mass ratio, pipe orientation etc.). In Figure 1 there are several types of horizontal wet gas flow pattern. The first is “Stratified Flow” (sometimes called “Separated Flow”). This is usually prevalent at low flow rates and low pressures. Here, the liquid flows at the base of the pipe (like a river) with the gas phase flowing
warning is given here that it is very rare for a wet gas industrial flow to reach the extreme conditions required for an orifice meter to behave with wet gas flow as it does with single phase flow. It should be noted that slug flow and a special pipe flow condition called “severe slugging” are not always the same phenomenon. Slug flow as described above, where the slug (i.e. the liquid mass that fills the cross section of the pipe) is caused by the interaction of the gas phase with an unstable liquid / gas interface can be relatively steady and the slugs only have moderate kinetic energy. It is unlikely that a slug flow pattern could cause significant damage to an orifice plate. However, “severe slugging” is a term often used that describes a different more problematic phenomenon. In industrial pipe flows that can have wet gas flows at low points in pipe work (or during shut down periods) excessive liquid quantities can gather in the pipe work. As the gas flowing is blocked by this occurrence the pressure behind the blockage can raise considerably and results in propelling the large liquid column or “slug” downstream at high velocity. The impact of such a slug with high kinetic energy on pipe work components can cause significant damage. Orifice plate meters have been buckled by such impacts. Figure 3 shows an example courtesy of Chevron Inc. Such slug strike damage on orifice plates has led to the “wok with a hole” meter phrase.
(Removing a buckled orifice plate from an orifice fitting can be a very time consuming and awkward task.) Orifice Plate Meter Wet Gas Flow Performance The original work on the response of orifice plate meters to wet gas flows was carried out by Schuster  in the late 1950’s. By the early 1960’s Professor Murdock  had written a seminal paper on the topic and then Dr Chisholm developed these ideas and published a series of technical papers on this and related subjects over a period of twenty five years from 1958 -83. Two of these papers [7, 8] are most important to researchers on the topic of wet gas metering with orifice plate meters. Much of the research up until 1977 was conducted by the power industry more focused on wet steam flow although there were wet natural gas flow data sets used by Murdock and Chisholm. After 1977 interest in wet gas flow metering seemed to wane but by the late 1980’s the oil and gas industry was reviving this old research and adopting it as the starting point for future research. In general though, the new researchers assumed with out evidence that orifice plate meters could not be good wet gas meters as the plate could potentially act as a dam to the liquid causing liquid hold and instability in the meter readings. However, none of the early research from the senior researchers Murdock and Chisholm ever suggested such a problem had been found with their comprehensive data sets. Nevertheless the oil and gas industries research concentrated on Venturi and cone type differential pressure meters. Rare, brief, descriptions of wet gas flow orifice plate performances at very low mass ratios were released by McConaghy  in 1989 and Ting  in 1995. These papers discuss the effect of extremely small mass ratios that are at or below the minimum values in Murdock’s and Chisholm’s data sets. McConaghy and Ting agree that for very small liquid content the gas orifice plate meter gives a lower gas flow prediction than the actual (i.e. the reference gas meter of their tests). Ting states: “McConaghy studied the effect of low liquid entrainment rate for 4” and 8” meters at β = 0.6. A relatively large under-measurement error of up to 1.0% was detected at a Reynolds number range of 3 to 8 million. Lower measurement errors were detected at β = 0.2.”
Figure 3. Orifice Plate Buckled by a Slug Strike While in Wet Gas Service. Buckled orifice plate meters have unknown single phase performance (as all plate damage is unique) and all wet gas flow performance is based on knowing the single phase base line performance. Also note that in natural gas production, wet gas flows tend to be flows with entrained particulates so wear of the sharp leading edge of the plate can be an issue as again, with a damaged plate orifice plate meters have unknown single phase performance and the base line for wet gas flow performance is lost. Therefore, it is important that the condition of the plate is checked regularly when in service with wet gas flow.
Figure 5. Sketch of Chisholm’s DP Meters Gas to Liquid Density Ratio Wet Gas Effect. Figure 5 shows a sketch of Chisholm’s findings at higher mass ratios. The x-axis is the Lockhart Martinelli parameter, i.e. for a set gas to liquid density ratio a direct measure of the liquid content in the gas, the higher the Lockhart Martinelli parameter the greater the liquid content of the flow. The y-axis shows positive bias on the actual gas flow rate due to the liquids presence. This is sometimes called the “over-reading”. Here, the uncorrected gas flow rate value predicted by the orifice meter is denoted as “ m g , Apparent ”. The “over-reading” of the orifice meter with wet gas is:
Over ? Re ading = m g , Apparent mg
Figure 4. Low Mass Ratio Flow Results Through 2” Orifice Plate Meters. Ting then showed wet gas data for 2” orifice plate meters with three beta ratios (β = 0.37, 0.54, 0.68). The test was with air and water at the low Reynolds number of 90,000 and the pressure low of 4 psig. The maximum mass ratio was approximately 0.13. Figure 4 shows the results. Like McConaghy, Ting found a small under-prediction of the gas flow rate (i.e. a negative bias usually now called an “under-reading”). The maximum under-reading was approximately -1.5%. It should be noted however, that due to the extremely low pressure the maximum Lockhart Martinelli parameter that the maximum mass ratio of 0.13 converts to is 0.005. Ting suggested such a wet gas flow is possible downstream of inefficient separators or if liquid was to condense out of a gas flow. Ting reported a visual description of the flow pattern: “No liquid accumulation in front of the orifice plate was observed for the beta ratios tested. Water streaks were seen flowing over the orifice plate to the other side.” That is, there was such a low liquid quantity that none of the above described flow patterns could form. Murdock and Chisholm tested orifice meters with much higher mass ratios / Lockhart Martinelli parameters and the results were different to those of McConaghy and Ting’s later low mass ratio tests. The Murdock and Chisholm tests were for 0.005 ≤ XLM ≤ 0.3, i.e. the majority of the data was for a mass ratio a magnitude higher than the McConaghy and Ting trace liquid tests. There is no over lap between Murdock and Chisholm’s earlier work and the work of McConaghy and Ting.
? . ? ? m g , Apparent ? Over ? Re ading % = ? ? 1 *100% . ? ? mg ? ? ?
Chisholm and Murdock stated that as the liquid content of a gas flow increased so did the over-reading of an orifice plate meter. Chisholm went on to show that for all other conditions held constant as the pressure increases (i.e. the gas to liquid density) the over-reading reduces. Recent research has shown the Chisholm wet gas orifice plate meter correction factor published in 1977 agrees extremely well with modern wet gas orifice meter data from CEESI. Figure 6 shows results for a 4”, 0.5 beta orifice plate meter tested with wet natural gas at CEESI.
CVX COP 4", 0.5 Beta Orifice Meter All Data with Separated DR g/l, Corrected by Chisholm
40 35 30 25 % Error 20 15 10 5 0 -5 -10 Xlm 0 0.05 0.1 0.15 0.2 0.25 -2% 0.3 +2% DR g/l 0.018 Uncorrected DR g/l 0.018 Corrected DR g/l 0.045 Uncorrected DR g/l 0.045 Corrected DR g/l 0.083 Uncorrected DR g/l 0.083 Corrected
Note that the over-reading originates from the fact the liquid in a flow of a set gas mass flow rate increases the DP read by the meter. That is, the DP is higher than that expected if the gas flow alone. It is very important meter users understand that wet gas causes unusually high DP’s and that the DP transmitters should be selected accordingly to avoid possible DP transmitter saturation. A good rule of thumb for wet gas flows with XLM ≤ 0.3 is to size the DP transmitters for orifice meters to be twice the upper range limit than would be expected for if the gas phase flowed alone. The Liquid Mass Flow Rate The Achilles heel of most DP meter wet gas corrections is the liquid mass flow rate is required to be known before it can be applied. The Chisholm correlation is no different. This information is not at all easy to come by in real applications. Two common methods for obtaining the liquid flow rate in natural gas production are to either use test separator history or apply a wet gas tracer dilution technique. Here tracer chemicals (usually fluorescent dyes) are injected into a wet gas at a known rate. At a distance judged suitably far downstream to have allowed full mixing samples are taken and the relative intensity of the liquid samples fluorescence to the injection tracer fluorescence indicates the liquid flow rate. Further details of this technique are out with the scope of this paper. Worked Example of Applying the Chisholm Correction The following worked example shows how the Chisholm wet gas orifice plate equation can be applied in the field for the case of a known liquid mass flow rate. An actual test point from a 4”, schedule 40, 0.5 beta ratio orifice plate meter tested at the CEESI wet gas test loop is used. This wet natural gas at 43.8 bara (635.3 psia) and 306K (910F) has a gas density ( ρ g ) of 32.9 kg/m3 (2.06 lbm/ft3) and a liquid density ( ρ l ) of 718.2 kg/m3 (44.84 lbm/ft3). Such information would be known to an operator in the field. The liquid injection reference meter states that the liquid mass flow rate is 1.98 kg/s (4.37 lbm/s). In the field this would be information obtained independently from the orifice plate meter and Chisholm equation (e.g. test separator history or tracer dilution techniques). The test facilities reference gas meter indicated the actual gas mass flow rate was 1.60 kg/s (3.53 lbm/s). This gas flow rate information would not be known to a meter operator in the field. The resulting differential pressure measured produced across the orifice plate meter by the wet gas flow was 44,160 Pa (177.6”WC). Applying the standard dry gas calculation method with this differential pressure gave a gas mass
Figure 6. 4”, 0.5 Beta Orifice Plate Meter Data. In Figure 6 the solid points are the uncorrected orifice meter gas flow predictions and the hollow points show the prediction after the application of the Chisholm equation:
m g Apparent
2 1 + CX LM + X LM
? ρg C =? ?ρ ? l
? 4 ? ρl ? ? ? +?ρ ? ? g
?4 ? ? ?
It can be seen that with out the Chisholm equation correction some of the higher Lockhart Martinelli parameter value data sets have over-readings up to approximately +35%. After the Chisholm correction all the data is corrected to less than ±5% error in the gas flow rate. The majority of the data is corrected to within ±2%. Chisholm produced his correction factor by modeling separated flow. It should be noted that the data that had a “corrected” result > +2% was for flow patterns that were annular mist flows and had DP’s > 500” Water Column (“WC). This is a large DP and in practice if an orifice meter reads DP’s > 500”WC the beta ratio should be increased to reduce the DP to < 500”WC. If this is done the Chisholm equation corrects the over-reading to give a gas flow rate within ±2% of the actual gas flow rate value. In Figure 6 the only exception to this rule is the lowest pressure (or gas to liquid density) tested. Here, it was the lowest velocities of the lowest pressure that gave the poorest results. A camera at CEESI identified the problem to be slugging flow in the test piece. That is, this is an unsteady flow and all meters would give some error under this inherently unsteady flow pattern. This is a poor flow condition to attempt to measure. Chisholm’s wet gas correction for orifice meters therefore worked if the DP’s were kept below 500’WC (a reasonable request in the field) and the wet gas flow was not unsteady. It should be noted however that Chisholm’s correction is formed from orifice meter in wet gas where the meter diameter is 2 ?” to 4”. There is little information on the effect of applying Chisholm’s correction to other diameters.
flow reading (i.e. m g , Apparent ) of 2.14 kg/s (4.72 lbm/s). Here we see that the presence of the liquid has caused the meter to over-read the actual gas mass flow rate. The difference between the meters uncorrected gas mass flow rate prediction and the actual (or reference) gas mass flow rate is 33.7%. Of course in the field the operator only knows the gas and liquid densities, the raw differential pressure and therefore the uncorrected gas flow rate prediction ( m g , Apparent ). He as yet does not know the actual gas mass flow rate. Now, if the liquid mass flow rate is available through an independent source the Chisholm equation can be applied to find the gas mass flow rate: The gas to liquid density ratio is:
Comments on the Published Research and his Worked Example Note the differential pressure measured was 177.6”WC. Steven et al  suggest the Chisholm equation is only applied if the differential pressure is less than 500”WC so this example is within this limit. Note that Chisholm created his correlation from orifice meter data with a diameter range of 2.5” to 4”. This example is for a 4” meter. The effect of extrapolating the diameter value in either direction is not well understood. Chisholm used multiple data sets with several types of liquid. It is current understanding of researchers that orifice plate meters are relatively resilient to the effect of changing the liquid properties on wet gas flow overreadings. Hall et al  showed 2”, schedule 80, 0.515 beta orifice meter wet gas data for natural gas with different mixes of water and hydrocarbon liquid. It was found that the liquid phase had little effect on the wet gas over-reading but all the 2” data (regardless of the ratio of water and hydrocarbon liquid) tended to agree and give a smaller over-reading than the data for the 2.5” to 4” data. This resulted in the Chisholm correlation over correcting the gas flow rate to give an under prediction. Figure 7 shows this result as shown by Hall.
Density Ratio =
2.06 lbm / ft 3 44.84 lbm / ft 3
Applying equation 3a gives:
? ρg C =? ?ρ ? l
1 ? 4 ? ρl ? 4 1 ?4 ? ? = (0.0458)4 + ? ? + ? = 2.62 ? ? ?ρ ? ? 0.0458 ? ? ? g?
Now with equation 1 used in equation 3 with this value of “C” we get:
200 psia Uncorrected 200 psia Chisholm Correction 400 psia Uncorrected 400 psia Chisholm Corrected 800 psia Uncorrected 800 psia Chisholm Corrected
mg = ? . ? ml 1 + C? . ? mg ?
m g Apparent
? ? . ? ? ml + . ? ? ? ? ? ? mg
? ? ? ? ?
30 20 10 0
+2% 0 0.1 0.2 0.3 -2% 0.4 Xlm 0.5
Now as we know m g , Apparent ,
ρ g , ρl , C
and if we
know the liquid mass flow rate m l we can iteration this Chisholm equation for the only unknown, i.e. the actual gas mass flow rate, m g . That is:
Figure 7. BP 2”, 0.515 Beta Orifice Meter Wet Gas Data from CEESI, Uncorrected and with Chisholm Correction. Hall pointed out that Chisholm’s correlation is a data fit and hence new data fits can be made on the model created by Chisholm for orifice meters out of Chisholm’s data range. For Hall’s very limited 2” data set it was found that the following correction factor worked well:
? ρg C =? ?ρ ? l ? 5 ? ρl ? 5 ? ? ? ? +?ρ ? ? ? g?
4.72 ? 4.37 ? ? 1 ? ? + ? 4.37 0.0458 ? 1 + 2.62? . . ? ? 0.0458 ? ? m ? mg ? ? g ?
The result is m g = 3.57 lbm/s (i.e. 1.62 kg/s). That is +0.9% from the reference gas meter compared to an uncorrected +33.7% from the reference gas meter.
Figure 8 shows the performance of equation 3b with the uncorrected data shown in Figure 7. However, Hall expressly stated that this was an example only from a small data set and users should apply it to a 2” orifice
meter wet gas flow situation at their own risk. More research is required to better understand the smaller orifice meters wet gas response.
200 psia Uncorrected
50 40 % Error 30 20 10 0 0 -10
200 psia Corrected 400 psia Uncorrected 400 psia Corrected 800 psia Uncorrected 800 psia Corrected
The best is widely considered to be the Chisholm correction factor but users should be aware that this is for DP’s ≤ 500”WC and for pipe diameters of between 2 ?” and 4”. For within this range and an accurately known liquid mass flow rate the Chisholm correction is said to give the gas flow rate to within ±2%. Use out with this range can lead to unspecified additional uncertainties. As yet no published correlation is widely accepted for use at diameters out with ?” and 4”. References 1. International Standard Organization, 5167 Part 2. 2. American Gas Association Report No. 3, Part 1. 3. Gas Processors Association GPA 8185-90, Part1. 4. American National Standard Institution ANSI/API MPMS 14.3.1 5. Schuster R.A., “Effect of Entrained Liquids On A Gas Measurement”, Pipe Line Industry Journal, February 1959. 6. Murdock. J.W., “Two-Phase Flow Measurements with Orifices" Journal of Basic Engineering, Vol.84, pp 419433, December 1962. 7. Chisholm D., "Flow of Incompressible Two-Phase Mixtures through Sharp-Edged Orifices", Journal of Mechanical Engineering Science, Vol. 9, No.1, 1967. 8. Chisholm D., "Research Note: Two-Phase Flow Through Sharp-Edged Orifices", Journal of Mechanical Engineering Science, Vol. 19, No. 3, 1977. 9. McConaghy, B.J., Bell D.G., and Studzinski, W., “How Orifice-Plate Condition Affects Measurement Accuracy”, Pipe Line Industry, December 1989. 10. Ting V.C. et al. “Effect of Liquid Entrainment on the Accuracy of orifice Meters for Gas Flow Measurement”, Int. Gas Research Conference, 1993. 11. Steven R., Ting F. & Stobie G., “A Re-Evaluation of Axioms Regarding Orifice Meter Wet Gas Flow Performance”, South East Asia Hydrocarbon Flow Measurement Workshop, Kuala Lumpur, March 2007. 12. Hall A. & Steven R., “New Data for the Correction of Orifice Plate Measurements in Wet Gas Conditions”, South East Asia Hydrocarbon Flow Measurement Workshop, , Kuala Lumpur, March 2007.
+2% 0.1 0.2 0.3 Xlm 0.4 -2% 0.5
Figure 8. BP 2”, 0.515 Beta Orifice Meter Wet Gas Data from CEESI, Uncorrected and with Altered Chisholm Correction. Finally it should be noted that little has been said with regards to vertical flow applications. Little information is in the public domain with regards to this. It is widely agreed no meter should be installed where there could be churn flow. However, annular mist flow installations are more appropriate but little to no data and correction factors has been published on the issue. Conclusions It is very important to regularly check the plate for damage when the orifice meter is in wet gas service. A damage plate must be replaced. All published research is for horizontal flow (although for various tap positions – whether an orifice meter has flange taps, corner taps or pipe taps appears to make little difference to the wet gas response). No information is available for vertical wet gas flow orifice meter installations. If an orifice meter has to be installed in vertical up wet gas flow it should at least be in a situation where annular mist flow exists and not churn flow. When in use an operator should check that the DP transmitter is not reading the upper range limit of the device. If it is the device is saturated and the DP read is not reliable. This is a very common problem with wet gas flow measurement with any DP meter – including the orifice meter. Trace liquids in a gas flow can cause an under reading of the actual gas flow rate by up to 2%. There are no correction factors available in the literature. Considerable liquid mass flow rate with the gas flow will cause an orifice meter to over-predict the gas flow rate. For a liquid mass flow rate known from another source there are correction factors published for horizontal applications.