Circulation Systems for Single and Multiple Seal Arrangements (Part One)


Written by:
Gordon Buck and Ralph Gabriel, John Crane Inc.

This discussion opens a three-part series covering mechanical seal piping plans that provide guidelines for various seal arrangements, fluids and control equipment to help you determine what support system requirements will maximize the performance reliability of your application.

Introduction

The American Petroleum Institute (API) created a numbering system for a variety of seal flush plans. The API flush plans are now located in API Standard 682 and the corresponding ISO standard, ISO 21049. The American National Standard Institute (ANSI) adopted a slightly different designation system.

General Information

These plans are utilized to provide the seal with the proper environment, depending upon the type of equipment used and the application the seal is exposed to. This series of articles discusses the basic flush plans, providing some general guidelines to be used along with the advantages/disadvantages of the plans, and, where appropriate, information on sizing and proper control of the system.

In this first part of the series, the basic concepts, criteria and considerations for circulation systems are established.

The various flush plans can be grouped by a variety of categories. One method for grouping is as follows:

api plan.jpgThese groups can have similarities in advantages/disadvantages, sizing of the system, and system controls.

Advantages and Disadvantages

The internal and recirculation systems have the advantage that the flush source comes from the pumpage and goes back to the pumpage, so no product contamination occurs. In addition, these flush plans, unlike an external injection, do not require any reprocessing of the product.

These same flush plans share the disadvantage that if the product pumped is not a good face lubricant, then the seal can become damaged. For some of the plans noted circulation from the pump discharge back to pump suction or vice versa will decrease pump efficiency and increase power required for the application. The volume of flush is usually very small compared to the capacity of the pump and, therefore, the decrease in efficiency is very small.

Sizing

Generally, the flush rate must be calculated based on fluid properties, system pressure, shaft speed, and seal size. See the "Flush Rates" section for more details.

System Control

The preferred method for controlling flow is with an orifice. The orifices should not be less than 1/8-in, unless the product is very clean and customer approval is obtained. Many small or low speed pumps have a low differential pressure and no orifice would be required in the piping. On the other hand, when the differential pressure is high, a single 1/8-in orifice would allow for more flow than desired.  In such cases, multiple orifices, choke tubes or valves must be used to control flow.

Flush Rates

With few exceptions, any flush system works hand-in-hand with the hardware and seal components. If the seal is set up with a distributed or single point flush, and/or an enlarged bore seal chamber, the effectiveness of the system will be better and the seal will run cooler no matter how much or little the flush flow rate is.

Flush requirements for seals should be given in terms of a minimum and a recommended flow rate. Some seals can actually operate satisfactorily without a flush. Such applications usually involve non-volatile fluids at low pressures and low speeds. Heat transfers from the faces, through the liquid and into the metal surrounding the seal chamber. Analysis of these cases is beyond the scope of this article.

The minimum flush rate is necessary to obtain the performance rating given by the product technical bulletin; it is determined by an energy balance computation. The assumption is that heat generated by the seal faces is absorbed by the flush through ideal mixing. This raises the temperature of the flush. Typically, an increase of 15-deg F for water and low vapor pressure hydrocarbons, 30-deg F for lube oils, and 5-deg F for  high vapor pressure hydrocarbons is allowed. Frequently, the minimum flush rate is relatively low, often less than 1-gpm.

Field experience indicates, and laboratory tests confirm, that seal performance generally improves when the flush rate is greater than the minimum. In particular, heat transfer usually improves and the average temperature around the seal decreases with increased flush rate; as a result, the face temperature and wear rate decreases. The recommended flush rate promotes these benefits.

The recommended flush rate should be based on experience with similar applications. Some considerations include performance goals and fluid properties as well as the design and interaction of the seal chamber, gland, flush plan and seal. In the absence of specific experience, a simple rule of thumb is: the recommended flush rate is the larger of 1-gpm per inch of seal size or the minimum flush rate.

 

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