February 2023

Digital Feature

Highlights of API 685 3rd Edition, Sealless Pumps—Part 3

API 685, “Sealless Centrifugal Pumps for Petroleum, Petrochemical and Gas Industry Process Service,” has been updated to the 3rd Ed. and was published in July 2022. It addresses both magnetic drive pumps (MDPs) and canned motor pumps (CMPs). This work (Part 3) focuses exclusively on secondary containment/secondary control, along with associated instrumentation requirements.

AUTHORS: F. Korkowski, Applied K3nowledge Consulting, Brea, California; T. Hess, E²G | The Equity Engineering Group, Inc., Philadelphia, Pennsylvania; J. Cooper, Bechtel Energy, Houston, Texas; and M. V. D. Heuvel, bp, Randstad, the Netherlands

API 685, “Sealless Centrifugal Pumps for Petroleum, Petrochemical and Gas Industry Process Service,” has been updated to the 3rd Ed. and was published in July 2022. It addresses both magnetic drive pumps (MDPs) and canned motor pumps (CMPs).

Due to the nature of the extensive work done to compose the 3rd Ed., there are three parts to this article. Part 1, published in November 2022, addressed the significant changes in API 685 3rd Ed. Part 2 covered the “other changes of interest to the reader” in understanding revisions from the previous API editions, including the influences and reasons behind each. This work (Part 3) focuses exclusively on secondary containment/secondary control, along with associated instrumentation requirements. The contents of this article were first published at the Turbomachinery and Pump Symposium held in Houston, Texas (U.S.) in September 2022.

Introduction. The two topics of instrumentation and secondary control/secondary containment go together since once it is decided whether secondary control or secondary containment is required for a sealless pump, then the appropriate type of instrumentation is applied. It is up to the user and the vendor—due to experiences or preferences—to determine what will be needed on a case-by-case basis.

Instrumentation and controls. The option to specify a bearing wear monitor was moved from the general protective instrumentation section to Section 9 for CMP because bearing wear monitors are not offered for MDP.

Pump protective instrumentation, including power monitoring, is now a bulleted item stating that the protective instrumentation will be installed by agreement between the purchaser and supplier. In API 685 2nd Ed., supply of the instrumentation with the pump was mandatory.

When instrumentation is specified, a discussion between the purchaser and the vendor shall address which of the following items must be included:

  1. Pump power or flow monitoring to indicate operation above or below the acceptable operating range or high power from decoupling the magnetic drive.
  2. The monitoring of leakage from primary containment to the secondary containment and control area.
  3. The monitoring of the containment shell or stator liner temperature or maximum liquid temperature in the rotor chamber.
  4. A sensor to detect liquid to prevent dry running or the pump being started with no liquid.
  5. The last item—liquid detection—is new to the 3rd It was added because sealless pumps contain bearings that are cooled and lubricated by the process fluid, so operation with no liquid in the pump will reduce bearing life.

A new bulleted paragraph was added for monitoring the containment shell (MDP) or stator liner (CMP) temperature by one of the following methods:

  1. A temperature sensor on the containment shell of a MDP between the drive magnet and the casing cover, or on the dry-side of the CMP stator liner.
  2. A means to measure temperature of the fluid in the rotor cavity for CMP.
  3. For fast-acting sensors, the temperature sensor will be attached to the containment shell of a MDP between the inner and outer magnet rings (the location between the inner and outer magnet rings on the containment shell will experience the most rapid temperature rise when dry running or when decoupling of the magnets occur).

The requirement for the protective instrumentation to be in accordance with API 614 and API 670 was also added.

The 2nd Ed. bulleted paragraph that specified two different methods to detect leakage from the primary containment into the secondary containment based on relative vapor pressure was modified. The 3rd Ed. states that the method to detect leakage will be agreed upon by the purchaser and the vendor, and that Annex R provides guidance on suitable instrumentation mounting and locations.

Annex R, “Instrumentation and Protective Systems,” provides a listing of the instrument abbreviations, descriptions and locations and includes functions and diagrams of MDPs and CMPs. A diagram of a vertical CMP was added to Annex R in the 3rd Ed.

Secondary control and containment. The secondary control and containment section of API 685 3rd Ed. was amended with a note to identify that secondary containment is a standard design for CMPs.

The bulleted paragraph requiring the vendor to state maximum flowrate from the secondary control system in the event of a primary containment failure was modified to state that the control system vent/drain piping shall be sized for the full flow of fluid through the rotor cavity.

A note was added to the bulleted paragraph for provision of drain connections for the secondary pressure casing. The note advises that the addition of drain connections may not be feasible for some CMPs because the connections may not have been included with the explosion protection certification.

The use of Annex P, “Hazard-Based Specification of Control/Containment Guidelines,” as a selection procedure for control containment options was changed from a bulleted choice to a note in the secondary control/containment section.

Annex P was updated for the 3rd Ed. and is a valuable tool for selecting applicable secondary control and containment methods for MDPs and CMPs. The informative annex is based on the “Hazard Statement Codes” according to the 2017 edition of the United Nations (UN) document ST/SG/AC.10/30 Rev 7, “Globally Harmonized System of Classification and Labeling of Chemical (GHS)” and the European Union (EU) Regulation (EC) 1272/2008. Annex P contains a table associating “H” or “EU” statement to a group code. The group code is used in the flowchart contained in the annex to select the secondary control and containment arrangement, as well as associated instrumentation.

FIG. 1.  This hazard statement label example is for methanol for the European community (EU). It reflects three hazard statement categories with codes that are used in Annex P. The lowest code in a category would always apply—in this case, Code H225 found in Group 1 and would require secondary containment.

TABLE 1. This table illustrates the relationship between the specific "H Statement" numbers (found in Table P1 of Annex P) and how they translate to either Group 1, 2, 3 or 4 (Group 1 is most stringent, Group 4 is least stringent within the specific parameters). The lowest Group number determines the instrumentation selection requirement codes: CMP-1, MDP-2, etc. for either CMP or MDP, accordingly.

TABLE 2. This table translates the instrumentation selection code (CMP-2, MDP-2, etc., from the previous table) into the listing of specific instrumentation required for both CMPs and MDPs.

For MDPs, several design and performance features were added to Section 9 regarding secondary control and containment. These include the following:

  • A requirement for a replaceable leakage restriction device of spark-resistant material requirement was added to the secondary control system for MDPs in the event of primary containment failure. Lip seals are not acceptable for this purpose. A double-wall containment shell can be used in place of the leakage restriction device. The leakage restriction device is required to meet local emissions regulations or exhibit a maximum screening value of 1,000 ml/m3 (1,000 ppm vol) as measured by EPA Method 21, whichever is more restrictive.
  • Secondary containment systems, which eliminate leakage around the outer magnet carrier shaft for MDPs, are required to include a suitable replaceable sealing device, such as a mechanical seal or double-wall containment shell. This sealing device must be rated for the same pressure as the pressure casing. If mechanical seals are used with secondary containment systems, they shall operate with a stand-by life of at least 25,000 hr without the need for replacement. If the pump is shut down after failure of the primary containment shell, the seal in a static condition must contain the fluid for a minimum of 24 hr in compliance with local emissions regulations. The design shall include a means to test the functionality of the mechanical seal, which is typically a low-pressure air test.

Practical example: CMP. Annex P provides excellent information in providing the logic diagram for determining whether secondary control or secondary containment may apply, along with recommended instrumentation; however, it is up to the user and the vendor, due to experiences or preferences to determine what will be needed on a case-by-case basis.

At the 2020 Turbomachinery and Pump Symposium, William Rademacher presented a tutorial based on his experiences at bp’s Whiting refinery where he installed a couple dozen CMPs (single-stage, multi-stage and vertical).1 The primary objective with sealless pumps, particularly CMPs, is to keep the pumping liquid as a liquid throughout the pump/motor internal circulation system since it is used for cooling, lubrication and thrust balancing. This is the reason why pump vendors provide the temperature/pressure profile in Annex K, which was improved to require the profile at specific areas within the pump and at-rated, maximum and minimum flows along with the liquid’s vapor pressure.

For the Whiting pumps, most of Annex P was used; however, certain additional helpful items were included. The most significant instrument used was a liquid level detector (tuning fork style) which was installed in the pump suction line piping after the block valve and suction strainer (and if a cooler is needed, in the area right above the cooler inlet) to prevent dry-running the pump (and cooler). The detector is interlocked with the motor starter to ensure that the pump is not operated unless the detector signals liquid is present in the suction piping. Check valves are used to provide liquid from the stand-by pump, while the use of a timer provides a time delay to cool off the motor while the process fluid condenses back to a liquid before restarting—all which prevents dry-running from occurring. 

Regarding keeping a pump from running below its minimum continuous stable flow (MCSF) to prevent thrust problems, minimum flow spillback lines were used. Another helpful instrument added was a reverse rotation indicator because, unlike a conventional pump with a mechanical seal and external motor, visual indication of pump rotation is not possible. As required by API 685, power or flow monitoring (installed in the switchgear), leakage monitoring [(pressure switch or transmitter (preferred)] in the pump’s secondary containment area, and temperature monitoring shell or maximum liquid temperature in the rotor chamber were installed. A bearing wear sensor (which API states “if specified” by the user) was included.

After the decision of whether secondary control or secondary containment is required for a given sealless pump and instrumentation requirements are established, the next area to address is the logic for instrumentation; that is, what should be set to alarm, trip and shut-down the pump. This could vary by site, by specific service, etc. As an example, the Whiting refinery’s sulfuric acid CMPs tripped the pump on loss of liquid level (using the turning fork level detector) in the suction piping, high motor winding temperatures (2 out of 3 voting) and fluid recirculation temperature. Tripping a pump due to a breach of the stator liner (sensed by the pressure transmitter) varies by user and even unit type. At the Whiting refinery, the breached pump continues to run until the standby pump comes online, as addressed in API 685 for secondary containment. For additional technical information (including some helpful photos) on the Whiting refinery example, along with tips on design, operation, installation and replacing conventional mechanical seal pumps with canned motor pumps, refer to the 2020 Turbomachinery and Pump Symposium tutorial.

Takeaways. To recap this three-part article on the highlights of API 685 3rd Ed., Part 1 covered significant highlights of API 685 3rd Ed.; Part 2 addressed most of the other changes of interest to the reader and this article focused on instrumentation and the subject of secondary control and secondary containment.

Where do we go from here? Do we keep API 685 as a standalone document or perhaps make it part of API 610? Do we expand API 685 to include overhung multi-stage sealless pump designs within the main body of the future standard? What new technology must be explored and applied to API 685?

The authors welcome all comments and suggestions for topics—within and beyond what has been addressed in this article—for additional consideration.


For all references and acknowledgements, refer to Part 1 of this three-part series.


1 Rademacher, W., “How to specify, design and install canned motor pumps in a refinery,” 49th Turbomachinery and 36th Pump Users Symposia (virtual), December 8–10, 2020, Houston, Texas.


Frank Korkowski (korkowskifrancis@gmail.com) is the Director of Engineered Training at Applied K3nowledge Consulting. He is a consultant recently retired from Flowserve and previously was the Marketing Manager for the API 1- and 2-stage process pumps. He spent 45 yr in various pump roles with Ingersoll Rand, Ingersoll-Dresser Pumps and Flowserve. Korkowski earned a BS degree in industrial engineering from the New Jersey Institute of Technology, with post-graduate studies in mechanical engineering and business administration at Lafayette College and Fairleigh Dickinson University. For the last 25 yr, he has served on API task forces API 610, API 685, API RP-691, and currently on API 682 and API 674.

Tom Hess (thess@e2g.com) is the Principal Rotating Engineer for The Equity Engineering Group, Inc. Prior to joining Equity, Hess worked as a Rotating Reliability Engineer in an oil refinery. He has been fascinated with sealless pumps for nearly 30 yr. He earned his BSME degree from Villanova University, is a member of ASME and is a registered professional engineer in the Commonwealth of Pennsylvania. Hess is a member of the API 685, 610, 682 and 613 Task Forces.

Jeremy Cooper (tjcooper@bechtel.com) is a Principal Rotating Equipment Specialist for Bechtel Energy Inc. in Houston, Texas (U.S.). Since 2001, he has worked as a Project Manager and Project Engineer with suppliers such as Flowserve and IFS. Prior to joining Bechtel, he was a Rotating Equipment Engineer at Fluor. Cooper currently serves as the Chairman of the API SOME, is the task force chairman of API 610 and API 685 and participates in several other task forces. He has worked startup and commissioning assignments in both international and domestic projects and is currently the Functional Lead for Rotating Equipment at Bechtel.

Martein van den Heuvel (Marteinvanden.Heuvel@bp.com) is a Senior Rotating Systems Engineer working for the central innovation and engineering team of bp. This year, he celebrated 30 yr of rotating equipment related experience in various operational, maintenance and engineering roles in the chemical and oil and gas industry. Heuvel received a BS degree in Mechanical Engineering from the Technische Hogeschool Rotterdam and a MS degree (honors) in maintenance management from the Caledonian University in Glasgow. He is a member of the API 685 taskforce. 

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