isa ad plowery final draft 3-24-09

Copyright 2009, Instrumentation, Systems, and Automation Society. All rights reserved. ISA 54th Analysis Division Symposium, 2009

NEW INTRINSICALLY-SAFE DIGITAL BUS ENABLED, MULTI-VARIABLE FLOW SENSING SOLUTION FOR ADVANCED PETROCHEMICAL

PROCESS ANALYTICS CONTROL AND MONITORING

Patrick Lowery P.E. John Dick Director of Technology Engineering Manager CIRCOR Tech CIRCOR Tech

KEYWORDS

Multi-variable Differential Pressure, Inferential Mass Flow, Volumetric Flow, Control Area Network, Intrinsic Safety, Thermal Mass Flow, Variable Area Flow Meters, Repeatability,

Process Analytical Technology, Sample Conditioning Systems

ABSTRACT Petrochemical facilities and associated process analyzer equipment are in need of robust and remotely-accessed flow metering/control instruments, mainly in process analytical (PA) sample flows; of which, require particulate tolerance, high-temperature and chemical resistance, and other conditions that require “ruggedness”, pressure/electrical safety, and signal repeatability. The technology presented in this paper is a new category of multi-variable differential pressure (MVDP) flow technology, developed for hazardous-area petrochemical low-flow monitoring/control along with a new intrinsically-safe (IS) digital CANbus (Control Area Network) architecture. Design and technical features of the MVDP sensor, new IS CANbus concepts, and flow performance data are presented and general comparison are made against existing technologies including thermal mass flow controllers (MFC’s) and variable area (VA) flow meters.

INTRODUCTION Petrochemical, specialty chemicals, biopharmaceutical, and other similar process industries have the need for low-flow monitoring and control. They are continuously searching for more robust low-flow


metering technologies which lower overall maintenance requirements, increase process uptime, or enhance various processes. The low-flow sensing technology search has been driven by various end users and ad-hoc user group initiatives such as the New Sensors and Systems Initiative (NeSSI™) and the Process Analytical Technology (PAT) initiative, which strive to bring efficiency and/or automation to process analytical systems and low-flow process control sub-systems. The process variables quoted as “mission critical” within both the NeSSI and PAT initiative committees are flow, pressure and temperature (1). Market studies and interviews of executives within the chemical process and petrochemical refining industries all agree that there will be an upcoming skilled labor shortage to run or maintain various processes (2). As a result of having to “do more with less” using remote monitoring and automation, the petrochemical process analytical industry is recently embracing the adoption of a new generation of digital electronic and field-bus monitored sample conditioning systems (SCS) which can monitor, alarm, and take action on certain events in a very cost effective manner as compared to traditional analog/mechanical systems. Being able to remotely monitor critical systems also has the intangible benefit of increasing safety for engineers and technicians by enabling them to spend less time in hazardous areas performing maintenance or manual monitoring tasks. For a petrochemical end user to leverage the benefit of new remote monitoring and control technologies, the core system architecture must be built from a foundation of robust sensors, valves, and various control or value-added sensor technologies. In addition, industries such as petrochemical must maintain very high safety electrical, explosive area, and pressure-containment specifications. These challenges create very large market barrier to entry for new technology. This paper presents a new type of low-flow, multivariate flow sensor which is a custom and miniaturized derivative of multi-variable differential pressure (MVDP) flow transmitter technology. The new MVDP instrument presented in this paper is based on a microelectromechanical system (MEMS) technology sensor along with new packaging, embedded signal conditioning, and new “low-overhead” intrinsically-safe digital communications. MULTIVARIABLE DIFFERENTIAL PRESSURE FLOW BACKGROUND Differential pressure or “orifice plate” flow meters are one the most common flow meter technologies in all of the process industries. The concept of differential flow measurement dates back to the days of the Roman Empire; however, the first modern MVDP flow meter was introduced by Bristol Babcock (now part of Emerson Process Management) in 1992. Therefore, MVDP is still categorized as one of the “new” flow meter technologies along with ultrasonic, Coriolis, magnetic, and vortex shedding.(3) MVDP technology integrates a static gauge pressure and temperature sensor measurement with a traditional differential pressure (DP) measurement across a flow restriction element to acquire upstream pressure and fluid temperature in addition to the differential pressure. The additional upstream fluid static pressure and temperature allows for various calculations which can be used to calculate gas density or can be used for various offsets to the DP measurement.


Figure 2 shows a graphical representation of a differential producing geometry showing the upstream and downstream pressures, densities, flow passage diameters, and fluid velocities.

ρf2

ρf1

Vf2

Pf1

dP

Plane 1

Plane 2

Pf2

Vf1

Uniform velocity

Uniform velocity

acceleration

velocity

h

datum

Diameter (D)Diameter (d)

FIGURE 1: GRAPHIC SHOWING A DIFFERENTIAL PRODUCING ELEMENT AND THE ASSOCIATED VARIABLES USED FOR FLOW CALCULATIONS. The rigorous derivation of the orifice equation starts with either the Navier-Stokes continuum mechanics equation or the subsequent Euler flow equation derivation in one-dimension. However, the most familiar derivation starts with the Bernoulli’s equation seen in equation 1.

constghVP f

f

f =++2

2

ρ (Bernoulli’s equation) (Equation 1)

Bernoulli’s equation represents an energy balance equation for the inertial (mass) energy (the pressure

Pf and density ρf components, kinetic energy ( 2/2fV ), and gravitational potential energy (change in

elevation or g*h). Although the complete derivation is not shown, the general orifice equation is derived from equating the sum of upstream kinetic and mass forces equal to the sum of the downstream kinetic and mass forces (while assuming no elevation difference between upstream and downstream pressure measurements). Since mass is conserved, a mass balance is created between upstream and downstream conditions and combined with the Bernoulli energy balance equation to finally arrive at a general form of the orifice equation seen in equation 2. (4)

[ ] PYfCdNdtdmm ∆

−==

•

)()(1 4

2

ρβ

(General orifice equation) (Equation 2)


Where: m = unit of mass = mass flux (mass per unit time (t)) d = Hydraulic diameter (effective diameter) of the orifice/ flow restriction D= hydraulic diameter of the main flow conduit before and after the orifice

∆P = Pressure differential across restriction (Pf1 – Pf2) β = beta ratio of the (d/D) diameter ratio C = discharge coefficient (empirically derived, but is the true flow rate/actual flow rate) Y = gas expansion coefficient (Y = 1 if fluid is liquid, 0<Y<1 for gases or vapors)

f(ρ) = function of fluid density (i.e. it is the actual density of the fluid if liquid and a derived density if gaseous phase) N = a geometrical unit constant which is dependent on what fundamental units are used.

The differential pressure in equation 2 is proportional to fluid kinetic energy loss from upstream to downstream; therefore, ΔP is proportional to the square of the velocity/flow rate. Hence, ΔP is the main parameter measured for pure volumetric flow rate calibration. The gas expansion coefficient (Y) and the density function [ ])(ρf are used primarily for gas flows or to convert volumetric flow to mass flow. The key elements of the MVDP measurement device are the DP Sensor, static pressure sensor, fluid temperature sensor, differential producing element or restriction element, pressure sensor tap conduits, and the main flow conduit in and out. The addition of static pressure and temperature sensors to DP measurement allow for the inferential calculation of a gas density based on either empirically derived electronic look-up tables or by calculation using various gaseous equations of state mathematical models. Fluid density can also be directly measured by use of a densometer sensor, primary restricted to only liquid density measurement. DP flow instruments can use a wide variety of geometries and mechanisms to create the differential pressure that is sensed during dynamic or flow conditions. These flow elements include orifice plates, laminar flow elements (LFE’s), venturi, wedge restrictions, Pitot static-tube, sonic nozzles, and various spring-loaded, V-cone, or other proprietary geometries. (5) TRADITIONAL OR LEGACY FLOW METERING TECHNOLOGIES FOR PAT AND SCS APPLICATIONS The petrochemical and related process analytical sub-markets use a wide variety of different flow measurement techniques to monitor or control analytical sample streams. These technologies include thermal mass flow controllers (MFC’s), variable area (VA) meters (commonly called rotameters), micro-Coriolis, and occasionally turbine or magnetic flow meters. Table 1 shows a general list of advantages and limitations of these different metering technologies when used in petrochemical process analytical applications. TABLE 1: TYPICAL FLOW METERING DEVICES USED IN PETROCHEMICAL PROCESS ANALYTICAL. (THE MVDP SENSOR IN THIS STUDY SITS IN THE “MIDDLE” RELATIVE COST CATEGORY.)

Technology type

Benefits Limitations Relative Cost


Coriolis

• Very high accuracy

• True mass/ density

• Multivariable

• Not intrinsically-safe (low flow versions, can be made EX proof) • Cannot measure low density gases • Particulate sensitive • Complex to maintain • Difficult to calibrate or re-scale

Most expensive

Magnetic • High accuracy • Particulate

tolerant

• Electrically conductive liquids only • Large in size, relative

Expensive

Thermal MFC’s

• Mass flow capability

• Low gas flow sensitivity

• Particulate sensitive • Gaseous only (typically) • Vapor condensation intolerant • Gas species specific, must re-calibrate to re-scale or change gas type • Intrinsic safety difficult

Middle

Turbine

• Widely available • Electronic

output • High pressure

capable

• Low flow gaseous meters are not common with turbine technology • Highly particulate intolerant • Stiction at low flow rates within range • Maintenance intensive

Below Middle

VA meters • Visual indication • Widely available

• Low accuracy • Typically require visual measurement • Liquid versions need to be armored • Remote outputs are limited to limit switches or highly inaccurate

magnetic sensors • High maintenance requirement • Pressure limited • Must be vertical in orientation

Least Expensive

THE CIRCOR Tech DMT-2000™ MVDP SENSOR AND DESIGN FEATURES The MVDP device of interest in this study (CIRCOR Tech DMT-2000™ series) is specifically designed for both liquid and gaseous applications in low-flow process control and process analytical systems which require hazardous area safety requirements. Process analytical sample conditioning systems (SCS) pose a unique challenge for flow metering applications. These challenges include:

• Relatively high fluid and ambient temperature conditions (60-80°C), • Entrained liquid droplets, • High probability of particulate contamination (catalyst residue, solid polymers, dirt), • Aggressive or corrosive fluids and related fugitive emissions (halogen compounds/acids,

hydrogen sulfide/sulfuric acid, caustics, highly adsorbent compounds), • Explosive vapors always present, must meet explosion proof or intrinsic safety specifications, • Low boiling point liquids and high vapor pressure (volatile) gases tend to boil or condense into

liquid when undergoing a pressure drop (Joule-Thompson heating or cooling). Figure 3 shows a cross section of the MVDP meter design with the key features indentified. The MVDP device incorporates several unique features which make the device suitable for a wide variety of low-flow process, liquid or gaseous, hazardous area applications. These features include a high pressure capable (600 or 1500 psig max operational pressure), MEMS multi-variable DP capsule, fast pressure-transient dampeners for dynamic pressure pulse protection of the MVDP sensor (i.e. hydraulic


hammer), modular restriction element (MRE), and liquid bypass valve for sudden liquid pulsations or transients. The truly enabling feature which allows for multi-variable data communication is a new intrinsically-safe, fault-tolerant data communication bus.

LIQUID HAMMER

SNUBBING PROTECTION

MEMS PRESSURE,

DP, AND TEMP SENSORS

HERMETIC SEAL ISOLATION BETWEEN

ELECTRONIC HOUSING AND FLUID HOUSING

EXCESS LIQUID

BYPASS VALVE FOR LIQUID

HAMMER PROTECTION

ALL ELECTRONICS ARE INTRINSICALLY SAFE

AND HIGH TEMPERATURE

CAPABLE

FIELD ACCESSABLE AND USER CONFIGURABLE AND APPLICATION SPECIFIC RESTRICTION ELEMENT INSERT

STAINLESS OR HASTELLOY METAL DIAPHRAGM AND HIGH PRESSURE ISOLATION MEMS CAPSULE

STAINLESS STEEL, IP-RATED

ELECTRONICS ENCLOSURE

DIGITAL AND HIGH-ACCURACY SIGNAL CONDITIONING AND

COMMUNICATION

FIGURE 2: CROSS SECTIONAL VIEW OF THE MVDP SENSOR AND ALL THE SALIENT FEATURES INCORPORATED FOR AGGRESSIVE LIQUID AND GAS FLOW METERING SERVICE. INTRINSICALLY SAFE CONTROL AREA NETWORK (CANBUS) One of the fundamental requirements of any multi-variable sensor application is the need to transmit several variables over two wires that can also meet the stringent electrical safety and redundancy requirements of hazardous area specifications. The quest for a “low overhead” (i.e. small and relatively inexpensive) intrinsically-safe digital communication standard began within the NeSSI™ community where several existing hazardous area-capable digital bus standards were reviewed including the Field Intrinsically-Safe Concept (FISCO) standards of Foundation Fieldbus™ and Profibus PA™. However, these pre-existing standards are capable of high level process control applications, and plant-wide process control optimization. They are also relatively expensive to implement and large in size.

Flow direction is from left to right. (Surface Mount, ISA-76.00.02 compliant version shown)


An industry consortium comprising of several analytical instrument OEM’s, instrumentation suppliers, CANbus electronic integrators, and industrial automation companies looked to develop an intrinsically-safe (IS) version of the Bosch 2.0b CANbus physical layer (i.e. the electronics hardware). This group drafted the first “IS capable” CANbus physical layer standard (CiA draft standard 103) through the CANopen international standard group called CAN in Automation (CiA). This IS CANbus runs at a relatively low baud rate (125k byte/sec), is limited to 10-15 meters of length, and uses special 9.5V IS power supplies. (6) The IS CANbus, using the CANopen™ or CIRCOR iCAN™ application protocols, comes with all of the core features of address conflicts, fault tolerance, network determinism, alarming, and other closed-loop control features. Figure 4 shows a physical example of an intrinsically-safe SCS with two MVDP sensors onboard using IS CANbus.

FIGURE 3: MODULAR GAS CHROMATOGRAPH SCS WITH TWO MVDP SENSORS FLOW CONFIGURATION MODULE (FCM) One of the new features of the MVDP meter, as compared to legacy DP meter or other technologies such as MFC’s, VA meters, or Coriolis, is the ability to change, re-scale, and custom tailor the FCM differential producing element in the field. The FCM can be tailored to meet many application requirements depending on whether the streams are clean or dirty, liquid or gas, pulsation or steady flow, or what magnitude of flow rate is needed. DP meters have not been used in low flow applications due to geometric, sensing, and rangeability limitations of the restriction and DP sensor. In

Two surface manifold mount MVDP sensors mounted on a modular SCS system.


order to create a restriction magnitude that creates a flow rate in the range of typical analyzer flow rates (e.g. 50 ccm for typical GC analyzer flow rates), the restriction must be on the order of ~0.005 in (0.127mm) with a DP somewhere around 1 psid (6.89 kPa). Such a small passage diameter has a high potential to clog as well as the DP sensor having less resistance to overpressure damage (DP pressure cells can only withstand a typical one sided overpressure of 4-10X of the maximum DP measurement range). The MVDP device can employ an FCM with a “stacked orifice” configuration where a series of discreet orifices are stack sequentially in series, but are arranged at 180 degrees opposite of each other. The principle of the stacked orifice configuration is to maximize the effective flow passage diameter (which will lower the probability of particulate clogging) while optimizing the differential pressure which can be sensed by the MVDP sensor for the given application. Figures 5 and 6 show computer simulations of pressure drops and flow trajectories and using a computational fluid dynamics (CFD) technique. Figure 6 shows an area of vortices/turbulence, downstream of each orifice stage, in which massive particles can “fall out” and accumulate due to loss of inertia. This sequence is repeated several times over and has the effect of an “inertial filter”. The combination of the expansions/contractions, turbulence, and flow directional changes result in a larger magnitude of differential pressure for a given flow rate. This allows a user to select a larger effective flow passage diameter (i.e. that a particle would “see”) than would be normally allowed using a single orifice. Table 2 shows empirical results of air flow with different sized orifice stacks with the orifice in-line and 180 degrees apart in a “zig-zag” pattern.

FIGURE 4: ZIG-ZAG RESTRICTION PATH SHOWING PRESSURE DROPS IN THE DIFFERENT STAGES


FIGURE 5: CFD FLOW STREAMLINES SHOWING VORTICES DOWNSTREAM OF EACH ORIFICE TABLE 2: COMPARISON OF PRESSURE DROP VS. REYNOLDS NUMBER & FLOW RATE AND THE INCREASE AND THE MEASURED “EFFECTIVE FLOW PASSAGEWAY DIAMETER” BY USING A STACKED ORIFICE ARRANGEMENT

orifice stack size (in)

max flow@5 psid (lpm air)

Reynolds number

% increase in effective flow

diameter0.0150 0.750 428.61 38%0.0250 1.810 1034.39 47%0.0320 2.810 1605.88 51%0.0500 6.040 3451.77 51%0.0600 8.216 4695.33 66%0.0150 0.800 457.19 25%0.0250 1.880 1074.39 45%0.0320 3.255 1860.19 40%0.0500 8.025 4586.17 40%0.0600 9.880 5646.28 51%

Zig-Zag

In-line


FIGURE 6: REPEATABILITY-HYSTERESIS DATA OF THE MVDP SENSOR ACCURACY PERFORMANCE DATA Flow outputs of the MVDP were compared against a flow calibration metering standard. Data was gathered to ascertain the limit of the MVDP sensor accuracy, resolution, and repeatability (hysteresis). Figure 7 shows the output of the scaled MVDP sensor as a function of applied differential pressure. The DP was increased to the full scale of the device then it was subsequently decreased to see whether the calibrated flow output of the sensor exhibited any hysteresis or non-repeatability. As seen, the outputs overlaid each other when put onto a common graph. The level of repeatability was below the accuracy output of the differential sensor ( 0.25FS). Further repeatability tests are needed with more accurate DP sensors to ascertain the detailed hysteresis characteristics.


FIGURE 7: MVDP SENSOR FLOW ACCURACY DATA SHOWING CALIBRATED METER OUTPUT AS COMPARED TO REFERENCE STANDARD MEASURED OUTPUT. Figure 8 shows the MVDP flow sensor output when compared to a reference standard. As seen, the %FS error falls well within a 1%FS accuracy band (the large hyperbolic shaped bands). A more rigorous analysis of the data (not shown) shows the MVDP sensor exhibits a 0.63% of reading accuracy with a 99.7% (3σ) level of significance. SCS applications tend to be demanding in overall wear and tear; therefore, the “ruggedness” of the sensor almost becomes a more important parameter than pure accuracy or repeatability. Table 3 shows the zero output drift of the raw MVDP sensor output with repeated simultaneous upstream and downstream pressure cycles from 0 to 600 psig over 250,000 cycles. As seen, the full-scale offset, after repeated cycles full pressure cycles, show a negligible effect on the sensor. In fact, the MVDP sensor is designed to withstand repeated common mode, 1500 psig cycles when the proper MEMS sensor die is used inside the sensor capsule. Only a portion of the available MVDP meter data is presented with respect to the balance of ruggedness and performance. Other tests, such continuous temperature cycling between sub-freezing and high temperature have been conducted and are still ongoing. Other tests that have been conducted are all of the pertinent hazardous area, pressure equipment directive, and digital bus electromagnetic immunity and radiative tests.


TABLE 3: OUTPUTS OVER REPEATED 600 PSIG COMMON MODE PRESSURE CYCLES CUMULATIVE CYCLES OUTPUT 1 (mV) OUTPUT 2 (mV) OUTPUT 3 (mV)

0 -0.4300 -0.6600 6.29001253 -0.5300 -0.9200 6.2100

10947 -0.0800 -0.3400 6.610037142 -0.5000 -0.8600 5.890076127 -0.3500 -0.8700 6.2700

110240 -0.1800 -0.4000 6.5200120000 -0.5200 -0.9700 6.4800130294 -0.3500 -0.7200 5.4700257288 -0.2600 -0.4600 6.3900

MIN -0.5300 -0.9700 5.4700

MAX -0.0800 -0.3400 6.6100

STDDEV 0.1580 0.2382 0.3562

NET CHANGE 0.1700 0.2000 0.1000

NET %FS READING CHANGE 0.004% 0.005% 0.002%

CONCLUSION The surprising accuracy of the MVDP sensor is leading to new applications as well as derivations of the pure flow monitoring device. These applications include inferential mass flow calculations, inferential thermodynamics such as Joule Thompson effects of high vapor pressure fluids, permeability and filtration monitoring, and closed-loop liquid and gas flow/pressure control. The pressure-based measurement technique utilizes very low power as compared to other methods which allow the MVDP sensor to be easily deployed in a variety of IS digital communication schemes, as well as traditional bus communication schemes. In summary, the new additional variables provided by MVDP sensor combined with the accuracy/repeatability performance should lead to incorporation of SCS into portions of the analytical method, such as validation, barometric pressure compensation, and physical property calculation. This will allow the end user to “push the envelope” with existing process analytical monitoring and control solutions.

REFERENCES 1. Data pertaining to market needs for new flow technologies was gathered from interviews with various Fortune

500 petrochemical companies, biotech, and high-pressure reactor researchers as well as interviews at NeSSi™ and ISA committee meetings from 2003 to present.

2. De Long, David W., “Chemicals Industry Leaders: Are You Ready For The Workforce of the Future?”, Changing Workforce Demographics, Issue Two, Accenture · Institute for Strategic Change, Nov. 25, 2001, pg 1.

3. Yoder, Jesse, “Pressure Transmitter Market Trends, From Globalization to New Technology”, Flow Control Magazine, August, 2004.

4. Miller, Richard, Flow Measurement Engineering Handbook, 3rd Edition, McGraw Hill, Boston, 1996, pp. 9.21-83.

5. Spitzer, D.W., Flow Measurement, 2nd Edition, ISA, 2001, pg. 135. 6. Can in Automation, Draft Standard CiA103, 2006, www.can-cia.org.

NeSSi™ is a trademark of the Center for Process Analytical Chemistry at the University of Washington CIRCOR Tech™ and CT76 iCAN™ are trademarks of Circor Instrumentation Technologies Foundation Fieldbus™ is a trademark of the Fieldbus Foundation Profibus PA™ is a trademark of the Profibus Organization

isa ad plowery final draft 3-24-09

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