Differential Pressure Flow Meters

A differential pressure flow meter calculates the volumetric flow rate by measuring the pressure drop across a precisely engineered flow restriction. The restriction may be an orifice plate, venturi tube, pitot tube, flow nozzle or other engineered obstruction. The technology is based on Bernoulli's Principle, which states that as fluid velocity increases through a restriction, pressure decreases proportionally. By measuring this pressure difference, the flow rate can be accurately calculated.

Modern differential pressure flow meters typically consist of three main components: the flow restriction (primary element), the pressure sensor or transmitter (secondary element), and flow computers or displays that convert the pressure measurements into meaningful flow data (tertiary element).

KOBOLD offers several differential pressure solutions including orifice plates (KPL and RCM), venturi meters (RCD), and pitot tubes (ANU). The technology offers exceptional versatility, measuring virtually any fluid - liquids, gases, or steam and excels in harsh industrial environments due to their robust construction, lack of moving parts, and proven reliability across extreme temperature and pressure conditions. They remain the preferred choice for many critical industrial applications due to their combination of accuracy, reliability, and cost-effectiveness.

Differential pressure flow meters offer several distinct advantages over other flow measurement technologies:

• Reliability & Longevity: Unlike mechanical flow meters with moving parts that wear over time, differential pressure meters have no moving components in the flow stream. This makes them exceptionally reliable in harsh industrial environments and reduces maintenance requirements significantly.
• Versatility: Differential pressure meters can measure virtually any fluid such as liquids, gases, steam, and even corrosive chemicals, making them one of the most versatile flow measurement solutions available.
• Proven Technology: Based on fundamental fluid dynamics principles, differential pressure measurement has been refined and proven as a very successful measurement technology. This extensive field experience means predictable performance and well-understood operating characteristics.
• Cost-Effectiveness: For many applications, especially larger pipe sizes, differential pressure meters offer the most economical solution with the lowest total cost of ownership when considering initial cost, installation, and long-term maintenance.
• High-Temperature/High-Pressure Capability: Unlike many electronic flow meters, differential pressure systems can handle extreme process conditions, including temperatures exceeding 1000°F and pressures up to 5800 PSI.

The main trade-off is that they do create some permanent pressure loss, unlike non-intrusive options.

KOBOLD manufactures several types of primary elements for differential pressure measurement:

Orifice Plates– A plate installed in the process line with a precisely machined opening. They provide highly repeatable pressure drops, are easy to install, and are available in a variety of materials for many different installation requirements, making them the most common and cost-effective option. KOBOLD offers a variety of different orifice styles to fit many applications such as:

• Concentric – Cylindrical opening with a sharp inlet and a conically tapered outlet.
• Bi-Directional – Cylindrical opening without inlet or outlet bevel to accommodate forward and reverse flows.
• Quarter Circle – Quarter circled tapered inlet for small Reynolds number flows.
• Conical Entrance – Conically tapered inlet and outlet for very small Reynolds number flows.
• Segmental – Half circle orifice that allows for better passing of slurries, liquids with solids, or liquids with gas bubbles.
• Eccentric – Off centered circular orifice that allows for better passing of slurries, liquids with solids, or liquids with gas bubbles.

Venturi Elements - Feature a gradually converging inlet, short throat, and gradually diverging outlet. This smooth flow transition creates only low permanent pressure losses and has excellent accuracy. Venturi tubes are ideal for applications where pressure loss must be minimized, such as large flow systems or energy-conscious facilities.

Pitot Tubes - Probe-style sensors that measure the difference between static and dynamic pressure, perfect for large ducts and minimal pressure drop applications. They are primarily used for gas or air flow measurements.

Each primary element of the KPL or ANU are paired with KOBOLD's PAD differential pressure transmitters to provide complete flow measurement systems. The RCD and RCM come with integral scale or transmitter options.

Other technologies on the market include flow nozzles, V-cone elements, and Wedge elements for specialized applications.

Differential pressure flow meters are fundamentally based on Bernoulli's principle and the law of continuity. Bernoulli's principle states that in a flowing fluid, an increase in velocity corresponds to a decrease in pressure. Differential pressure flow meters create a controlled restriction in the pipe, forcing the fluid to accelerate through the smaller cross-sectional area. This acceleration causes a pressure drop that is directly proportional to the square of the flow velocity.

KOBOLD's differential pressure meters precisely measure this pressure drop using highly accurate sensors. The relationship between pressure drop and flow rate follows well-established engineering equations, allowing for accurate flow calculation across a wide range of conditions. This fundamental principle makes differential pressure measurement reliable and predictable for a wide range of fluid applications.

Flow calculation from pressure drop follows established engineering principles based on Bernoulli's equation and the law of continuity.

Bernoulli's Equation: P₁ + ½ρV₁² + ρgz₁ = P₂+ ½ρV₂² + ρgz₂

Continuity Equation: ρ₁V₁A₁ = ρ₂V₂A₂

Where P = pressure, ρ = density, V = velocity, g = gravitational acceleration, z = elevation, and A = cross-sectional area.

When properly combined and solved for flow rate the basic relationship is that flow rate is proportional to the square root of the differential pressure. The simplified equation is Q = K × √(ΔP), where Q is flowrate, K is the calibration constant, and ΔP is the differential pressure. The calculation of the calibration constant accounts for the Reynolds number, pipe roughness, temperature expansion, fluid density, and other pertinent factors for accurate flow measurement. KOBOLD transmitters can perform the calculation from the measured pressure differential internally and provide direct flow output signals.

Yes, KOBOLD's differential pressure meters are capable of measuring gas and steam flows, and they are often chosen for these applications due to some key advantages:

• Cost effective for large pipe sizes
• Suitable for a wide variety of gases and steam (when compensated for pressure and temperature variations)
• Minimal maintenance requirements

One key consideration when selecting a differential pressure flow meter for gas or steam applications is that unlike liquids, densities can vary significantly with temperature and pressure changes. For accurate gas and steam measurement, proper selection and calibration are essential as the flow meter must be configured for your specific operating condition. If density changes do occur, real time compensation using external temperature and pressure sensors to determine the actual fluid density and apply a correction factor to the reading is needed. If dealing with variable density, KOBOLD offers the DVE and DVH, Multivariable Vortex Flow Meters, with optionally integrated temperature, pressure, and density readings for automatic compensation for processes with variable density.

Yes, both density and viscosity affect the reading of differential pressure flowmeters.

Density:

Fluid density has a direct impact on the flow measurement as the flow rate is inversely proportional to the square root of the density. This means that if the fluid density is different from the reference density the meter was designed for, the readings will be inaccurate without compensation. This is especially important for gas flow measurement, as the density of gases can vary significantly with temperature and pressure changes. If your process density varies significantly from what the meter was designed for, you may need to use external pressure and temperature readings to calculate the fluid density and apply a correction.

Viscosity:

Viscosity has a less pronounced, indirect effect on the flow measurement. Viscosity directly affects the Reynolds number which influences the calibration constant included in the flow equation accounting for real-world flow effects. The effects of viscosity are nonlinear and harder to correct for. As the viscosity increases the Reynolds number decreases, potentially leading to transitional or laminar flow, which can cause inaccurate readings.

For high viscosity liquids, explore KOBOLD's lineup of positive displacement flow meters, such as the DON or OVZ, that are specifically engineered to handle the challenging conditions of high viscosity fluids.

Yes, differential pressure meters require straight pipe runs to ensure accurate measurement by providing stable, fully developed flow profiles. Requirements vary by the type of primary element used and heavily depend on upstream piping configuration. KOBOLD's KPL orifice plates straight run requirements are based on DIN EN ISO 5167-2, while venturi meters (RCD) and pitot tubes (ANU) requirements can be found in the product documentation. The RCM requires at least 10x pipe diameter from any upstream with valves being recommended to be installed downstream of the flowmeter.

If adequate straight runs aren't available, flow conditioning plates (ZUB-FCZ) or flow straighteners (ZUB-FSB) can be used. KOBOLD's engineering team can evaluate your specific piping configuration and recommend the best solution to achieve accurate measurement within your space constraints.

Selecting the right differential pressure flow meter depends on several key factors:

• Application Requirements: Flow range, accuracy needed, and fluid properties (liquid, gas, or steam)
• Piping Considerations: Pipe size, available straight runs, and pressure drop limitations
• Fluid Properties: Viscosity, density, corrosiveness, and cleanliness

KOBOLD's Product Selection:

• KPL orifice plates: Best for large pipes and general applications
• RCM meters: Ideal for high flow rates and industrial monitoring
• RCD venturi meters: Perfect for small diameter pipes and low pressure drop requirements
• ANU pitot tubes: Excellent for large ducts and minimal pressure drop

The PAD transmitter provides the secondary element for KPL and ANU systems. Contact KOBOLD's technical team for application-specific recommendations and sizing calculations. You can schedule a free engineering consultation with one of Kobold's application engineers to discuss your requirements and get assistance specifying a flowmeter for your process.

KOBOLD offers comprehensive pressure tap and manifold configurations designed to optimize measurement accuracy, simplify installation, and facilitate ongoing maintenance for differential pressure flow measurement systems. The selection and configuration of these components significantly impact system performance, reliability, and maintenance requirements throughout the system lifecycle.

Pressure Taps:

Standard pressure tap options for orifice flange unions to be used in conjunction with orifice plate (KPL) installations include:

• Flange taps: Located 1” before and after the orifice
• Corner taps: Located immediately before and after the orifice
• D-D/2 taps: Located one pipe diameter upstream and ½ pipe diameter downstream of the tap

These pre-tapped orifice flanges reduce field installation labor and provide precisely tapped systems for accurate measurements.

The ANU pitot tubes feature integrated pressure taps designed for optimal measurement accuracy.

Manifold Systems:

For manifold systems, KOBOLD provides valve manifolds that allow for transmitter isolation, calibration, and maintenance without process interruption in various styles and configurations:

• 3-way Manifold Valve: Block and equalizing valves
• 5-way Manifold Valve: Block, equalizing, and vent valves

These valves are available for remote mounting and connection through impulse lines or direct mounting. Special materials and pressure ratings are available for demanding applications.

Most modern differential pressure flow measurement systems require a transmitter to convert the pressure difference into a usable output signal.

KOBOLD's PAD differential pressure transmitters are specifically designed to work with the different primary elements KOBOLD offers such as the KPL or ANU. The PAD provides several critical functions: it amplifies the small pressure signals, linearizes the square-root relationship between pressure and flow, and outputs standard industry signals (4-20 mA or HART).

The RCD and RCM meters are available as direct-reading devices with local indication options, which may be sufficient for some monitoring applications. They can also be specified with transmitters. The RCD is available with analog or switching outputs and the RCM is available with analog, switching, frequency, or HART® outputs.

Yes, KOBOLD manufactures differential pressure meters for virtually any pipe size. The KPL orifice plates are custom manufactured for specific pipe diameters and flow conditions, making them ideal for large industrial piping systems. RCM meters are available in standard sizes up to 8 inches, with custom sizes possible for special applications. The ANU pitot tubes can be manufactured to fit any pipe diameter and are particularly cost-effective for large duct applications. RCD venturi meters are available in standard NPT sizes from ½" to 3", with larger custom sizes available. All KOBOLD differential pressure systems include proper sizing calculations and flow coefficient determination to ensure optimal accuracy for your specific application and pipe size.

Yes, KOBOLD specializes in differential pressure meters for demanding industrial applications. The systems can be manufactured in various materials including 316 or 316L stainless steel, bronze, Monel®, or other exotic alloys for specific chemical resistance. High-pressure applications are accommodated through robust construction and appropriate material selection. The PAD transmitters are designed for heavy-duty industrial use and can handle extreme process conditions. Special diaphragm seal materials and process connections are available for corrosive applications. KOBOLD's engineering team works closely with customers to specify the proper materials and construction for each specific application, ensuring long-term reliability in challenging environments.

Yes, KOBOLD's PAD differential pressure transmitters offer HART protocol communication for configuration and diagnostics over the 4-20 mA signal wires. The transmitters provide digital outputs for flow rate, totalized flow, diagnostic information, and alarm status. The microprocessor-based design allows for flexible calibration, user-configurable parameters, and integration with modern control systems

Proper installation is critical for accurate differential pressure flow measurement. Key requirements include:

• Straight Pipe Runs: Ensure adequate upstream and downstream straight pipe sections as specified for your primary element type
• Primary Element Installation: KPL orifice plates must be installed with the proper flow orientation and with proper gasket sealing. ANU pitot tubes should be aligned with flow direction, installed at the correct depth in the pipe, and properly supported.
• Pressure Tap Installation: Use proper impulse line sizing and routing to minimize measurement errors. Slope lines appropriately for liquid or gas service
• Transmitter Mounting: Install PAD transmitters close to the process taps to minimize capillary effects. Provide easy access for calibration and maintenance
• System Commissioning: Perform proper leak testing, zero calibration, and span verification

KOBOLD provides detailed installation instructions and technical support to ensure proper system setup and optimal performance.

Regular calibration helps maintain measurement accuracy over time. Although KOBOLD's differential pressure systems are inherently stable due to their proven measurement principles, establishing regular calibration intervals ensures that measurements are accurate and reliable.

There is no standard calibration interval; the calibration frequency should be determined based on application severity, accuracy requirements, and process needs. Calibration should be carried out by comparison using a device that is notably more accurate. For critical applications, dual transmitters or periodic flow verification may be appropriate.

Impulse lines (also called sensing lines) are typically required to connect the primary element's pressure taps to the differential pressure transmitter. Proper impulse line design is crucial for accurate measurement. Key considerations include:

• Line Sizing: Usually ¼" to ½" tubing
• Routing: Lines should be sloped properly (up for gas service, down for liquid service) to prevent accumulation of foreign materials
• Material Selection: Compatible with process fluid and operating conditions
• Length: Keep as short as practical to minimize lag time and temperature effects

The KOBOLD RCD and RCM come as integrated devices and do not require additional impulse lines. Some KOBOLD configurations allow for direct mounting of PAD transmitters to minimize impulse line requirements. The ANU pitot tubes can be designed with integral pressure connections to reduce external piping. Proper impulse line design and installation are essential for system accuracy and reliability.

Unstable readings in differential pressure flow meters typically result from:

• Flow Disturbances: Insufficient straight pipe runs, upstream obstructions, or turbulent flow conditions.
  • Solution: Verify straight run requirements or add flow conditioning
• Process Issues: Pulsating flow, cavitation, or flashing in the primary element.
  • Solution: Damping in the PAD transmitter or process modifications
• Installation Problems: Partially plugged impulse lines, air bubbles in liquid service, or condensate in gas lines.
  • Solution: Proper line maintenance and purging procedures
• Electrical Interference: Poor grounding or electromagnetic interference affecting the PAD transmitter.
  • Solution: Proper shielding and grounding practices

KOBOLD's technical support team can help diagnose specific problems and recommend appropriate solutions based on your application and installation conditions.

When no differential pressure is detected, systematically check:

• Flow Conditions: Verify that flow is actually present and within the expected range
• Primary Element: Check for damage, improper installation, or blockage. Orifice plates can be installed backwards, get damaged with improper use, or become blocked
• Pressure Connections: Inspect for plugged impulse lines, closed valves, or leaking connections
• Transmitter Configuration: Verify the PAD transmitter is properly configured for your application range and zero point
• Electrical Connections: Check power supply and signal wiring to the transmitter
• Process Conditions: Confirm that process temperature and pressure are within design limits

KOBOLD can provide troubleshooting advice and technical support to help resolve measurement problems quickly and effectively.

Several methods can verify differential pressure meter accuracy:

• Calibration Verification: Use the PAD transmitter's built-in diagnostics and HART communication to verify zero and span calibration without process interruption
• Comparative Measurement: Install a temporary reference flow meter or use a portable ultrasonic flow measurement for comparison
• Material Balance: Compare flow measurements with tank level changes, pump curves, or other process data
• Primary Element Inspection: Verify that orifice plates are undamaged, pitot tubes are clean, and all components are properly installed

KOBOLD's engineering team can assist with accuracy verification procedures and provide guidance on maintaining optimal measurement performance throughout the system lifecycle.

Lead times for KOBOLD differential pressure flow meters vary depending on the specific configuration:

• Standard Products: RCM and RCD meters with standard specifications typically ship within 2-6 weeks
• Custom Primary Elements: KPL orifice plates and ANU pitot tubes require custom manufacturing and typically take 5-7 weeks
• PAD Transmitters: Standard configurations are stocked for immediate shipping. If not available in stock the typical lead time is 3-5 weeks.
• Special Materials or Configurations: Systems requiring exotic materials or custom engineering may take 6-8 weeks or longer
• Complete Systems: Integrated packages with multiple components may require additional coordination time

Expedited delivery may be available for urgent applications. Contact KOBOLD's sales team early in your project planning to confirm availability and delivery schedules. Providing complete specifications and application details helps ensure accurate lead time estimates and eliminates delays due to missing information.

Yes, KOBOLD provides comprehensive technical support throughout the entire project lifecycle:

• Application Engineering: Expert assistance with meter selection, sizing calculations, and system design to ensure optimal performance for your specific requirements
• Installation Support: Detailed installation instructions and technical drawings
• Ongoing Technical Support: Phone and email support for troubleshooting, maintenance questions, and system optimization
• Documentation: Comprehensive manuals, specification sheets, and certification documentation

KOBOLD's experienced engineering team understands the complexities of differential pressure flow measurement and can help ensure successful project implementation. Contact our technical support team early in your project to take advantage of our expertise and ensure optimal system performance.