In the oil, gas, petrochemical, chemical, and refinery industries, transporting fluids with specialized properties presents a significant technical challenge. Often, the fluids used are highly viscous, contain suspended solids, or are shear-sensitive, rendering centrifugal pumps ineffective. Therefore, positive displacement pumps, such as monoblock pumps, are used.
To ensure reliable, safe, and long-term operation of these pumps, certain design and manufacturing standards must be adhered to. One of the most important standards is API 676 , developed by the American Petroleum Institute and considered the most important design standard for positive displacement circulating pumps.
This article provides an in-depth look at API 676, its specifications, its relationship to individual pumps, the benefits and challenges of implementing the standard, and practical tips for selecting pumps based on the standard.
Screw pumps of the CFS-Inox series
Definition and purpose of API 676
API Standard 676, “Rotary Positive Displacement Pumps,” specifically regulates positive displacement pumps. This standard establishes minimum requirements for the design, manufacture, materials, installation, and operational testing of these pumps to ensure their reliability and operational safety in critical industrial environments.
The purpose of this standard is to create a common language between pump buyers and manufacturers to avoid technical disputes, premature failures and operational risks.
In fact, API 676 sets standards that ensure high-quality design, precise operation, long service life, and easy maintenance.
API 676 Scope
This standard applies to the following types of positive displacement circulating pumps:
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Gear pumps with internal and external gearing
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Camshaft pump
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Screw pumps
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Vane pump
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Single-chamber pump or step-chamber pump
However, this standard does not apply to piston pumps or variable displacement pumps (e.g. API 675).
API 676-compliant pumps are used to transport heavy and sensitive liquids in numerous industries, particularly in refineries, pipelines, and the production of paints, adhesives, pulp, and animal feed.
Single pump position in API 676
A single-screw pump (also called a screw pump) is a positive-displacement rotary pump consisting of a screw impeller and a rubber stator. The rotation of the impeller ensures a continuous flow of fluid from the inlet to the outlet.
Due to its performance characteristics, such as slow fluid transfer, consistent pressure maintenance, and the ability to pump viscous liquids and solids, the pump falls into the API 676 pump category.
According to the standard, single-stage pumps must be compatible with other rotary pumps in terms of casing design, material selection, seal type, vibration resistance and performance testing.
Design requirements in API 676
1. Aircraft design and main components
The pump casing must be sufficiently strong to withstand the operating pressure and the stresses of the piping. Its design must prevent fluid leaks and keep the internal pressure within acceptable limits.
All parts that come into contact with fluids must be made of corrosion-resistant, temperature-resistant, and abrasion-resistant materials. For petroleum-based fluids, stainless steel or carbon steel with a protective coating is typically used.
2. Install the base (base plate)
The pump base must be robust and designed to ensure precise alignment between the pump and motor. The base should have a slight slope to facilitate the drainage of escaping fluid and be equipped with drainage holes.
For large projects, the foundation structure must absorb vibrations and prevent the transmission of forces to the pipeline in accordance with API 676.
3. Bearings and shafts
The bearings must be designed for a minimum service life of 25,000 hours. The pump shaft must be made of high-strength steel and withstand the bending and torque generated by the fluid pressure.
For single pumps, the design of the bearings and the selection of the appropriate coupling are particularly important due to the wavelength.
4. Sealing system
Pump seals are a critical design element. API 676 favors mechanical seals because they are less permeable than packings and easier to maintain.
Under special conditions, for example in the presence of abrasive liquids, double sealing systems or special seals filled with protective liquids are used.
5. Motor and connection
API 676 recommends the use of flexible metal couplings to maintain pump and motor alignment and reduce vibration.
The motor or drive must have sufficient power to operate the pump under maximum pressure and flow conditions.
Job requirements
1. Flow rate and working pressure
The pump must deliver the expected flow and pressure within a specified range with minimal fluctuations. Performance testing involves comparing the actual flow with the design value and verifying the permissible error.
2. Vibration and sound
Pump vibration must not exceed permissible limits. Excessive vibration may indicate improper installation or alignment, as well as mechanical problems.
The noise level must also be within acceptable limits to prevent vibration transmission to the system.
3. Leaks
During the leak test, all connections, flanges, and seals must be tight. For pumps containing hazardous fluids, special sealants must be used.
4. Final exam
API 676 defines a series of factory tests that include:
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Hydrostatic pressure test
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Functional tests (flow and pressure tests)
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Vibration and noise testing
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Leak test
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Conducting the test
Material requirements in API 676
The materials used to manufacture the components that come into contact with the liquid must be compatible with its chemical and physical properties.
The following combinations are typically used for single pumps:
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Rotor: stainless steel or hard chrome-plated steel
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Stator: Durable rubber (NBR, EPDM or FKM), suitable for different temperatures and fluid types
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Valve body: carbon steel, stainless steel or cast iron alloy.
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Seal: silicon carbide or tungsten carbide , wear-resistant
All metal components must undergo a material identification (PMI) test to ensure alloy compatibility.
Advantages of API 676-compliant pumps
1. Increased reliability:
Precise design and high-quality control enable the pump to operate in harsh industrial environments and reduce downtime.
2. Long service life and low maintenance costs:
By selecting suitable materials and basic design, the service life of the pump can be extended and maintenance costs reduced.
3. High safety in hazardous environments:
Minimal leakage and stable operation reduce the possibility of danger to the environment and people.
4. International standardization:
The introduction of API 676 makes different design groups more compatible.
5. Special fluid applications:
Pumps that meet this standard are ideal for viscous, corrosive, or solids-containing fluids.
Challenges in implementing API 676
1. Increased production costs:
Compliance with all technical requirements, strict quality controls and special materials increase the final price of the pump.
2. Complexity of custom pump design:
Due to the rotor length and the temperature and wear sensitivity of the stator, accurate design according to API 676 requires a high level of knowledge and experience.
3. Accurate and stable installation is required: Small errors in the alignment or installation of the pump can
cause vibration and lead to long-term failures.
4. Compliance with other standards:
For some projects, local standard requirements may not be consistent with API 676, requiring careful engineering analysis.
The role of API 676 in quality assurance and inspection
Documentation is critical in the manufacture and delivery of pumps. API Standard 676 requires manufacturers to provide the following documentation:
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Design and assembly drawings
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Results of the functional and hydrostatic tests
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Material certificate
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Quality control report
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Installation and maintenance instructions
To ensure that the pump fully meets the standard requirements, a final inspection is usually carried out in the presence of a representative of the buyer.
Select a pump according to API 676
To select the correct pump according to API 676, the following steps must be followed:
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Fluid type analysis:
includes temperature, density, viscosity, chemical composition, and presence of solids. -
Determine flow rate and operating pressure:
To select the optimal pump for your needs, you need precise information about your process. -
Choose the right material:
Rotor, stator and cover materials are determined by the type of fluid. -
Check the ambient conditions:
Ambient temperature, altitude and installation conditions can affect pump performance. -
Review the standard conditions and identify exceptions:
If a deviation from a specific provision of the standard is necessary, it must be documented and approved by the employer. -
Factory Test (FAT):
Before final delivery, the pump must be tested at the factory and the results documented. -
Proper installation and operation:
Proper installation, accurate alignment, and vibration testing are essential for long-term operation.
Tips for maintenance and operation
API 676 recommends the following to extend pump life:
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It is necessary to measure the vibration and noise levels regularly.
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Make sure the bearings are properly lubricated.
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Seals and connections should be checked at regular intervals.
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Avoid running the pump dry.
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If the pump is not used for a long period of time, it should be cleaned and serviced with a suitable liquid.
Finally
API 676 is one of the most comprehensive and authoritative international standards for the design and manufacture of positive displacement pumps. Compliance with this standard in the design and manufacture of each pump ensures high quality, safety, and durability under harsh operating conditions.
Pumps that comply with this standard play a critical role in industrial processes due to their consistent performance, consistent flow, and ability to pump specific fluids . While fully compliant with the standard is more expensive, it reduces downtime, extends equipment life, and saves money in the long run.
Therefore, selecting a single pump that meets API 676 standards for critical projects is a sound engineering decision.