Flow Ratio Assurance: Much More Than a Flow Meter

  Spray foam insulation is a widely accepted method for insulating residential and commercial properties. Spray foam provides superior insulation, making homes and buildings more energy efficient, meeting demands and complying with new building codes that require stricter, more energy-efficient standards. Spray foam is produced by combining two liquid chemicals: isocyanate (A) and polyol resin (B). After these chemicals are mixed, they are sprayed onto a surface. The mixing of these two chemicals creates an instantaneous chemical reaction. When these two liquids are mixed rapidly, they typically expand between 10 and 50 times in size and fully react within seconds, becoming the final spray foam product. Most other construction products such as drywall, fiberglass insulation, oriented strand board (OSB), tiles, lumber and ductwork are manufactured in a factory and sent to the job site; however, spray foam is produced on site during application. This allows the materials to be sprayed as liquids, expand completely and fill voids, gaps, spaces around pipes, wires and tight areas, providing better insulation. Since it is produced on site, it is important to apply proper controls to ensure it is produced correctly.

Correct Equipment Selection Is Critical

The equipment required to properly mix and spray these chemicals must be capable of heating and pressurizing the materials. Additionally, the equipment must have a robust mixing method to ensure a homogeneous mixture. Proper on-site mixing of the two components requires professional equipment. Today, most spray foam chemicals require a 1:1 ratio to achieve proper mixing and reach the optimal properties that material manufacturers target in their chemicals. When applied correctly, spray foam provides many benefits; however, when applied incorrectly, it can lead to problems that are difficult and expensive to solve. The best action to take is to prevent problems from occurring. For this reason, relying solely on the applicator to ensure the foam is produced correctly is no longer sufficient. Construction professionals are adopting spray foam use in their homes and increasingly more homeowners are becoming knowledgeable about spray foam; these individuals seek assurance that spray foam application in their homes is performed correctly. At this point, it is important to have spray foam equipment designed to minimize the risk of "bad foam" spraying. In addition to the system's ability to detect possible issues in the equipment, process and chemicals, it must also be able to record and present data in a usable format when requested by customers. Graco's spray foam equipment is designed with robust design and software that monitor and control pressure and temperature values to reduce potential issues. The equipment also has a design that will alert the operator and shut down the machine when potential issues are detected. While Graco equipment is designed to help prevent "bad foam" spraying by detecting possible equipment issues, many problems associated with bad foam stem not from equipment issues but from factors under the control of the insulation contractor. These factors include improper preparation of chemicals or use of an oversized mixing tank for the feed systems. Mechanical equipment will also require preventive maintenance over time and equipment may experience problems requiring repair. Considering all these reasons, having equipment capable of detecting possible ratio error issues becomes important.

What Are Single-Point Variables?

It is important to understand the types of issues that can cause incorrect ratio foam spraying. These issue types are called single-point variables and can be divided into seven categories. • Air in liquid flow, • Feed pump smaller than required, • Poor material feed to the proportioner, • Proportioner pump issues, • Liquid leaks, • Liquid restriction in heated hose or spray gun. When these different single-point variables are understood, separate detection methods can be designed for each. When the variable type is selected, the variable can be monitored. This aims to monitor each of these variables and, when any is detected, shut down the proportioner, thus preventing incorrect ratio foam spraying. The operator can then make necessary adjustments or perform maintenance work to eliminate the issue causing the ratio error condition.

Graco's Ratio Assurance System

No single method can easily and correctly detect every potential single-point variable. A robust ratio control system must be comprehensive and have more than just flow meters. At the foundation of the system are mechanically linked pumps; followed by positive displacement piston pumps, pressure monitoring and flow meters providing ratio assurance with built-in features that deliver unique results in detecting ratio error conditions.

Mechanically Linked Pumps

At the foundation of all Graco Reactor products are mechanically linked pumps; this includes pumps in all electric, hydraulic and pneumatic reactors. The term "mechanically linked pumps" simply means that the A and B pumps are connected to each other by a shaft or coupling, so that both pumps perform strokes at the same speed and equally. When pumps are mechanically linked, for every cycle of pump A, pump B must also cycle. This forces the pumps to cycle at equal rates and in the same manner, enabling the pumps to pump proportionally. Graco believes that mechanically linking A and B pumps will always provide a robust system designed to spray at a 1:1 ratio. Mechanically linked pumps are, in a sense, built-in flow meters where the pumps naturally dispense equal amounts of A and B chemicals with each stroke. Since the ratio is constant, mechanically linked pumps provide consistent ratio within a tight tolerance range. Additionally, mechanically linked pumps are not dependent on flow meters to pump proportionally. A mechanically linked pump is automatically designed to dispense equal amounts of both A and B materials.

Positive Displacement Piston Pumps

The type of pump used for spray foam and coating applications is also important. The positive displacement piston pump is a proven design that Graco considers the best pump type for this application. A positive displacement pump moves liquid by holding a fixed amount and forcing the held volume into the discharge line (displacement). Positive displacement piston pumps provide consistent volumetric performance across a wide range of temperature, pressure and thus density ranges. Piston pumps are better suited for start-stop applications and maintaining standby pressure. Piston pumps can maintain the correct volume per cycle during extended use even with aggressive liquids. Graco's piston pumps have a precision machined using advanced CNC machining equipment; pumps are subject to very tight tolerances ensuring consistency between pumps. This becomes important when two pumps in a system must be relied upon for equal material volume. Graco maintains the tolerance between pumps at less than 1%.

Inlet Pressure Monitoring

Monitoring changes in inlet pressure provides a quick and reliable way to detect potential feed pump and material supply issues that could lead to incorrect material ratio. Inlet pressure monitoring is a standard feature on Graco's Reactor 2 elite models. When inlet pressure is monitored and falls below an acceptable pressure level, an issue can be detected and the user alerted. While flow meters and discharge pressure monitoring can also detect feed-related issues, inlet pressure monitoring is the most accurate and fastest responding detection method. Some of the most commonly encountered issues include exhausted chemicals, cold chemicals or undersized feed pump(s) relative to demand; all of these are best detected by utilizing inlet pressure monitoring.

Discharge Pressure Monitoring

Discharge pressure monitoring is standard on all electric and hydraulic reactors. Graco always leverages the pressure differential between A and B chemicals to detect and prevent incorrect ratio spraying operations. In reactors, the default setting for the pressure differential alarm is 35 Bar (customers have the option to adjust this value in the manner most suitable to their needs). When the pressure differential between A and B chemicals exceeds 35 bar, the Reactor will shut down. Using pressure monitoring has always been the best way to detect most ratio error conditions. While this basic rule works in most cases, there are some exceptions to this rule. Discharge pressure monitoring can also help detect conditions that can lead to poor impingement mixing of A and B chemicals. Poor impingement mixing can occur even when chemicals are in the correct ratio. Possible causes of impingement mixing problems include a clogged gun filter and/or clogged impingement connection points in the gun's side seals. These types of issues will cause one of the chemicals to increase in pressure, affecting the impingement mixing. As the pressure differential between A and B chemicals increases, complete impingement mixing becomes more difficult. Discharge pressure monitoring can detect these types of issues when the pressure differential exceeds the alarm threshold and shuts down the machine to prevent distribution of incorrectly mixed materials.

Flow Meters

Flow meters can detect specific conditions that lead to incorrect ratio distribution that cannot be detected by inlet or discharge pressure monitoring alone. Flow meters perform best in detecting issues related to proportioner pumps, air in feed lines/systems and some liquid leaks. Adding flow meters to the robust, mechanically linked positive displacement piston pumps and inlet and discharge pressure monitoring features that form the foundation of the reactor provides additional ratio assurance protection to the system. Flow meters connect the entire system by providing the capability to measure, monitor and record the correct volume amounts of A and B materials. When you know the actual volumes dispensed, this data can be made available for presentation to the customer. In the Graco Reactor 2 ratio assurance system, oval gear flow meters are used. Oval gear flow meters provide numerous advantages such as cost effectiveness, accuracy, ease of installation and versatility. Oval gear flow meters are generally considered one of the most cost-effective options for liquid flow management. This type of flow meter is ideal for managing liquids with various densities and high flow rates. The gear meters used in the reactor, calibrated at the factory, provide an accuracy level of ±1%. Ease of installation is another advantage of the oval design. Since there is no need to use a straight pump or prepare flow, oval gear meters can be installed in tight spaces where alternative technologies have failed. Oval gear flow meters are also an excellent choice for numerous industrial applications including chemicals, petrochemicals, water, oils, diesel fuel, paint, coatings, grease and solvents. Oval gear flow meters are designed to be simple and robust. Two oval gears meshed at 90 degrees rotate in a chamber of known volume. As these gears rotate, the outer oval shape of the gear and the chamber walls precisely and repeatedly fill and empty a known liquid volume. Each complete 180-degree rotation of the gears is called a pulse. The flow rate is then calculated based on the number of recorded pulses. Tom Vandevenne - Product Sales Specialist - Product Marketing Specialist - Graco