Porosity measurement of press felts on the paper machine (HSPT) - Technical Information Paper

Sep 15, 2026

Porosity measurement of press felts on the paper machine (HSPT) presents a standardized test procedure for measuring relative airflow through a felt running on a paper machine. It explains how HSPT readings relate to void volume and water removal, and how vacuum developed in the tester head reflects relative felt porosity. The paper outlines safety precautions, a summary of the miniature suction-box concept, and guidance for using HSPT for monitoring felt filling, air porosity profiles, and end-of-life prediction.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages3

Reading Time8 min

AccessFree to download

Scope

Correlation and use in monitoring

Safety precautions

Summary

Predicting time to batch wash and end of felt life

Comparing two or more felts

Irregularities

Crown vs. load

Localized fill up

Wear streaks

TIP 0404-29 OLD NUMBER 014-58 ISSUED – 1983 REVISED – 1990 REVISED – 1997 REVISED – 2004 CORRECTION – 2005 2004 TAPPI The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published. Porosity measurement of press felts on the paper machine (HSPT) Scope This Technical Information Paper1 describes a test procedure for the measurement of relative airflow through a felt running on the paper machine. Correlation between Huyck-Smyth Porosity Tester (HSPT) readings and void volume (1) and between HSPT readings and water removal (2,3) have been reported. This tester has been used by numerous mills to monitor changes in felt porosity throughout its life. Safety precautions Standard mill safety procedures and personal protection equipment should be properly utilized when taking measurements with the HSPT. Special care should be given to handing air line in the press section around moving components. Content Summary The porosity tester is essentially a miniature suction box used under standardized conditions for indicating the relative porosity of running felts. This property is measured by the vacuum developed in the tester head as a result of pressure drop through the felt. There are other state ofthe art instruments which are used to measure the performance capability of wet felts. One such device is the Feltperm. This instrument uses water permeability, and pressure differential through the felt to measure the resistance to water flow into the felt. Both porosity and permeability measurements are a function of felt design, degree of filling, level of compaction, and felt density. The operation of the Felt Perm unit, and a more detailed discussion of water permeability in wet felts can be found in TAPPI TIP 0404-43 . Both the felt’s air porosity and water permeability characteristics can be used during monitoring to indicate the felt’s ability to absorb, carry, and release water. The HSPT may not detect small areas that are plugged or narrow bands of wet streaks in the machine direction, which may be detected by the feltperm. However, it does provide an economical means of measuring general cross direction porosity profiles. Both instruments are useful tools in assisting the papermaker with monitoring felt filling, 1This TIP was previously issued as T 680 su-71 under the same title. TIP Category: Automatically Periodically Reviewed (Ten-year review) TAPPI 5 / Porosity measurement of press felts on the paper machine (HSPT) TIP 0404-29 Predicting time to batch wash and end of felt life Where felt fill-up is the determining factor in felt removal, H-S number vs. days life curve may be used to predict the end of felt life once the critical H-S number for the position and felt design has been established (Fig. 8).The critical H-S number is defined as that at which crushing, marking, or other adverse conditions associated with lack of felt porosity occurs. Experience has shown that after initial major changes have taken place in felt openness, further changes often become linear with time. When the curve begins to straighten at about half-life, it can be extrapolated to the critical H-S number for that position and an end-of-life prediction made. This allows the papermaker to schedule felt changes in coincidence with wire changes or other mechanical shutdowns and also to assist him in efficient scheduling of critical grades. Based on previous felt performance, a sub critical H-S number can be based on some percent below the critical HS number. The desired percent in reduction could vary depending on the felt and the stage in the felt life. For examp

FAQs

What does the HSPT test procedure measure on a paper machine?

It measures relative airflow through a felt running on the paper machine under standardized conditions.

How are HSPT readings used to predict felt life?

By using the H-S number versus days life curve to extrapolate to a critical H-S number associated with loss of porosity, enabling end-of-life prediction.

What safety precautions are recommended when using the HSPT?

Use standard mill safety procedures and personal protective equipment, with special care when handling air lines near moving components in the press section.

Sep 15, 2026

Uploaded by Felix Montgomery