KPL Orifice Plates and Orifice Flanges for Differential Pressure Flow Measurement
The KOBOLD KPL is an orifice plate primary element that creates a differential pressure reference for measuring the flow of liquids, gases, or steam in medium to large industrial piping. It is supplied as a plate alone or as a complete orifice flange union, in sizes from 2" to 24" (DN 50 to DN 600), and is designed to pair with a differential pressure transmitter such as the KOBOLD PAD.
With no moving parts in the flow stream, the plates require little or no maintenance.
How Orifice Plate Flow Measurement Works
When a pipeline is reduced at one point by a cross-sectional constriction, the flow speed of the medium increases at that point. By Bernoulli's energy equation and the law of continuity, the total flow head stays constant, so the increase in velocity causes a drop in static pressure. That pressure drop is the differential pressure head, and it is a measure of the flow rate.
Flow rate (q) is a function of the square root of the differential pressure (Δp): q = c√Δp, where c is the flow coefficient set by the orifice shape and the operating data. The medium must be single phase in a full, circular pipe; DIN EN ISO 5167-2 does not apply to pulsating flow.
Pairing the KPL with the PAD Differential Pressure Transmitter
A complete orifice plate flow measurement assembly combines the KPL primary element with a differential pressure transmitter that converts the pressure difference across the plate into a standard output signal. To obtain a linear flow signal, KOBOLD recommends a transmitter with internal square-root extraction, such as the PAD Heavy-Duty Industrial Differential Pressure Transmitter.
Compact versions are delivered with the KPL mounted together with the DP transmitter, manifold, and condensate pots; remote versions connect the transmitter through impulse lines. Mounting-position codes cover liquids (L1 to L8), gases (G1 to G8), and steam (S1 to S0). For steam, condensate pots keep hot steam away from the transmitter and keep the impulse tubing full.
Orifice Bore Designs and Reynolds Number Ranges
Six bore designs let the plate be matched to the medium and the flow conditions. Concentric plates are the standard design; the other designs cover low Reynolds numbers, bidirectional flow, and media that carry solids or gas bubbles.
- Concentric (standard): Re > 5000 for 0.1 < β < 0.56; Re > 16000 β² for β > 0.56
- Bidirectional: cylindrical bore for alternating flow direction; limits as concentric
- Quarter circle nozzle: 500 < Re < 5000
- Conical entrance: 80 < Re < 500
- Segmental: Re > 5000; segment-shaped opening passes solids or gas bubbles
- Eccentric: offset circular bore, same purpose as segmental; limits as concentric
Specifications, Pressure Ratings, and Materials
The KPL is rated to PN 420 or ASME Class 2500, approximately 6,090 psi (420 bar). The remote version is rated to 930 °F (500 °C); the compact version to 392 °F (200 °C) for liquids and gases and 572 °F (300 °C) for steam, higher on request. Sustained pressure loss depends on the orifice ratio d/D and is approximately 20 to 70% of the differential pressure head.
- Nominal sizes: 2" to 24" (DN 50 to DN 600)
- Flange ratings: ASME Class 300 to Class 2500, raised face or RTJ; DIN PN 16 to PN 100
- Orifice plate: stainless steel 1.4404 (316L); other materials and coatings on request
- Flanges: carbon steel (A105) or stainless steel 1.4404 (316L), weld neck as standard; slip-on and threaded on request
- Plate roughness: Ra < 32 µin (0.8 µm); installed concentric with the pipe
Pressure Tapping Options
Orifice flange unions have two ½" NPT taps per flange, 180° apart, one plugged. Flange tapping places the taps 1" (25.4 mm) before and after the plate and is preferred wherever ASME applies; corner tapping, preferred wherever DIN EN applies, taps immediately before and after the plate. D-D/2 tapping and angle taps are available on request.
Installation and Flow Conditioning
Straight run requirements follow DIN EN ISO 5167-2. Where upstream straight length is limited, a 19-tube bundle flow straightener (ZUB-FSB) or a ZANKER flow conditioning plate (ZUB-FCZ, for β ≤ 0.67) can be installed at least 7.5 D upstream of the plate. Accessories include manifold valves, condensate pots, an oval flange, and mounting brackets.
Ordering and Application Guide
Every KPL is calculated to DIN EN ISO 5167-2 (API AGA-3 on request) for its service conditions, so a completed Application Guide (Application Data Sheet) is required before a quotation or order unless the order repeats a previously specified product. Order codes cover the orifice plate alone (KPL-B), the flange union with plate (KPL-F), and the flange union without plate (KPL-N). Optional certificates include Material Certificate 3.1 per EN 10204 and NACE MR-0175/ISO 15156 material.
Common Questions about KPL - Orifice Plates for Differential Pressure Flow Measurement
The PAD turns the pressure drop at the plate into a standard transmitter output. Since flow is proportional to the square root of that pressure drop, the PAD's internal square-root extraction delivers a linear flow signal. On compact assemblies the PAD mounts on the manifold above the orifice flanges; remote assemblies connect it through impulse lines.
Line sizes run from 2" through 24" (DN 50 through DN 600). Weld neck orifice flanges to ASME B16.36 are offered in Class 300, 600, 900, 1500, and 2500 with raised face or ring type joint, and to EN 1092-1 Form B1 in PN 16, 25, 40, 63, and 100. The maximum rating is PN 420 or Class 2500.
Liquids, gases, and steam, provided the medium is in a single phase and the pipe runs full. Pulsating flow falls outside DIN EN ISO 5167-2. For liquids with gas bubbles, order a vent hole; for gases with condensation, order a drain hole. Both options are only available for plates mounted in horizontal pipes.
Standard concentric plates cannot be used at small Reynolds numbers, for example in oil measurement. The quarter circle nozzle covers 500 < Re < 5000, and the conical entrance orifice covers 80 < Re < 500. Both use corner taps only, with d > 15 mm (quarter circle) or d > 6 mm (conical entrance) and D < 500 mm.
Roughly 20 to 70% of the measured differential pressure, set by the orifice ratio d/D. Example, water at 41 °F (5 °C), d = 0.6 D, 4" (DN 100): 52.8 to 264.2 GPM (12 to 60 m³/h) gives 6.5 to 164.9 inH₂O (16.2 to 410.7 mbar) differential and a sustained loss of 2.8 to 72.3 inH₂O (7.0 to 180.1 mbar).
Flange tapping is preferred wherever ASME applies, with built-in taps that reduce field welding and drilling on the flow line. Corner tapping is preferred wherever DIN EN is valid, or when the flange leaf cannot be drilled, as with PN 6. Tap hole diameters are ¼" for 2" and 2½", ⅜" for 3", and ½" for 4" and larger.
The medium and its state (gas, liquid, or steam), pipe inner diameter or line size and schedule, operating pressure, temperature, viscosity, and density, and minimum, normal, and maximum flow values. KOBOLD returns the orifice bore size, differential pressure at each flow, beta ratio, discharge coefficient, and permanent pressure loss on a calculation sheet that accompanies every plate.
Condensate pots in steel A105 or stainless steel 316L, each 19.3 in³ (316 cm³) and about 3.7 lb (1.7 kg). They are filled with water before commissioning and kept at the same level to avoid inaccurate readings. Remote-mount 3-way and 5-way manifold valves in 316 stainless steel are rated 6,000 psig at 100 °F (410 bar at 38 °C).
Product Specifications
Maximum Pressure
420 Bar (Class 2500 tmax +500 °C )
Maximum Temperature
930 °F (500 °C) remote version; 392 °F (200 °C) liquids and gases, 572 °F (300 °C) steam, compact version
Process Connection
DN 50 ... DN 600, 2" ... 24" ASME Flanged
Wetted Material
Stainless Steel, Carbon Steel, Others
Media Categories
Liquid, Gas, or Steam
Product Features
Multiple Configuration and Design Options
Low to Moderate Pressure Loss
Cost-Effective and Rugged
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