This technology is key for localizing valve leakage.
by Pål Jacob Nessjøen, National Oilwell Varco Norway AS
March 24, 2014

As an integral part of onshore and offshore drilling, mud pumps circulate drilling fluids to facilitate drilling oil and natural gas wells. Mud pumps stabilize pressure and support the well during the drilling process. The drilling fluids reduce friction and remove drill cuttings. A team from a mud pump manufacturer created a leak detection system for hex pumps. A hex mud pump has six pistons, six suction valves and six discharge valves. The six pistons are driven by a rotating, asymmetric cam. The leak detection system monitors the suction and discharge valves using accelerometers.

The Case for an Automated Leak Monitoring System

Valve leaks in piston pumps are often discovered at a late stage when the leaks are so severe that they induce large discharge pressure fluctuations and create washout damage. When a severe leak is detected, operators localize it manually by listening to the fluid modules while the pump operates. However, this technique makes uniquely localizing the leak and distinguishing between a suction valve leak and a discharge valve leak difficult.

Test stand pumping systemImage 1. Test stand pumping system

Human exposure to hazards is the main disadvantage of manual detection, verification and localization. Mud pumps convert large amounts of power and often output high pressures of up to 350 bar. Additional equipment in pump rooms also generates high acoustic noise pressure levels that can exceed 100 decibel A and cause health and hearing damage if humans are not correctly protected (see Image 1). Valve leaks often develop quickly, so manual detection gives very little time to prepare for exchanging the defective valve(s) after the leak is detected. If the leak source is uncertain, searching for the defective valve(s) can be costly and time-consuming. To overcome these drawbacks, the mud pump manufacturer needed a remote system to detect and localize pump leaks.

Discovering the Vibration Method

During a vibration monitoring project for hex pumps, the team discovered the possibility of detecting leaks using accelerometers. At different locations on the pump and on the discharge line, vibrations, suction pressure, discharge pressure and pump speeds for different pump conditions were recorded. Team members used a 20-kilohertz (kHz) sampling frequency and recorded five-second snapshots with intervals of a few minutes. On one occasion, the vibration signature significantly changed during a 15-minute period. The team soon realized that the hotspot was a growing valve leak. After the initial discovery, team members performed more tests to explore the leak detection possibility, which results in vibrations of all six valve blocks when discharge valve 2 (D2 valve) has a severe leak. The trace numbers identified the accelerometer/valve block number. The high intervals of the dashed help curves represented the theoretical suction phases that occurred when the suction valves were closed. These curves offered easy interpretation of the vibration signals and were derived from the proximity of the sensor signal. The low values of the help curves represent the theoretical closing of the discharge valves, which happens when the respective pistons retract. The leak intervals had a lag-time shift relative to the theoretical intervals. This time shift was on the order of 25 milliseconds and came from the following:

  • Valve inertia causing delayed valve closing
  • Fluid compressibility causing a finite piston stroke to compress and decompress the fluid

Analyzing the frequency spectra indicated that the leak induced strong, broad-banded noise from 3 kHz up to the Nyquist frequency of 12.5 kHz (half the sampling frequency of 25 kHz). The overall noise level increased by a magnitude of 30 decibels.

Leak Detection System

Based on that encouraging experience, the team wanted to include this condition-based maintenance system as a standard feature on all its hex pumps. The system was a stand-alone module to add to any existing hex pump control system (see Image 3). Slightly simplified, it included the following components:

  • Accelerometers (one per valve block)
  • Proximity sensor that picked up pump speed and phase
  • Discharge pressure sensor
  • Embedded monitoring system with acquisition modules for powering the accelerometers and acquiring high-frequency data
  • Signal processing software
  • Alarm logics implemented using software running on the monitoring system
  • Human machine interface

The data acquisition and signal processing are briefly described with the following:

  • Capture high-rate of data (25-kHz sample rate) from all sensors during a short time interval covering at least one pump cycle
  • Bandpass filter the acceleration signals to minimize the influence from ambient pump vibrations
  • Analyze the timing signal to find the pump speed and cam angle
  • Construct adjusted window functions that selectively pick the filtered acceleration signal in every valve closing phase (In this case, “adjusted” means narrowed and time-lag corrected so that valve closing spikes are excluded.)
  • Use these windows to calculate the root mean square vibration level for each valve closing phase
  • Normalize the vibration levels through division by the median vibration level
  • Set a leak alarm if one or more of the normalized vibration levels exceed a specified threshold during a certain time interval

The default sampling frequency of the signals is 25 kHz, but the system can handle higher rates if necessary. The bandpass filter is optional, but experience shows that it improves contrast and detection sensitivity. Signal strength normalization by the median vibration level makes the detection nearly independent of the inherent ambient vibrations, which increase rapidly as pump speed and discharge pressure rise. The last requirement—that the detected leaks last for a set time—eliminates erratic alarms caused by debris or large particles that can cause temporary seal malfunction. The team can remotely verify the leaks detected automatically by signal processing in several ways. First, the operator can view and interpret the vibration signals directly from graphs. Second, the operator can selectively listen to the recorded acceleration signals as audio signals to hear the leak sound. Third, the operator can check to see if the mean discharge pressure is stable or dropping. Last, the operator can see if the lowest pressure harmonics are growing. The human ear/brain is an extremely sensitive instrument for detecting abnormal sounds. If the leak sound is too far up in the treble frequency range to hear, the team can play the signal back with a lower sampling rate. This transforms the leak noise into a more audible frequency band for the human ear.

Vibration signals<br />
with no leaksFigure 1. Vibration signals with no leaks

The desktop application shown in Figure 1 allows the pump manufacturing team to review the leak detection system and read raw logs and trend files directly from the leak detection system. This additional feature gives the operator the chance to get a closer view of the vibrations and perform audio playback to the user. Also, the team and end users can view the high-rate log of the discharge pressure to reveal a cyclic variation drop. This helps provide a better understanding of the valves’ operation. Figure 1 shows a 1.5-second snapshot of the vibration signatures after a severe leak developed in the D3 valve. It shows the filtered vibration signals from all six accelerometers. Acceleration Signal 3 has enhanced noise amplitude during the D3 phase. A closer look at the other signals revealed that the leak induced vibrations were transferred to the other accelerometers during the same time intervals. However, the vibration transfer was relatively low, actually less than -20 decibels for the neighboring valve blocks and even less for the other blocks, so vibration transfer is not a serious problem in hex pumps.

Conclusions

Based on the field experience of the new leak detection system, the team concluded that the leak detection method offers many advantages when compared to other techniques, including:

  • High sensitivity for early leak detection and localization
  • Remote, continuous and computer-based pump monitoring
  • Increased safety through less human exposure to hazardous environments
  • The detection and localization of multiple leaks (in hex pumps)
  • Reduced maintenance time and cost because leaky valve(s) are localized before the valve exchange jobs start
  • Easy to retrofit the system to existing pumps because accelerometers can be attached using glue, magnets or tape

The monitoring system and software proved to be fast tools for prototyping the system and gave the team an embedded deployment system that it can reliably retrofit to existing pumps. In comparison to other leak detection methods, which are based on analyzing discharge pressure, the vibration-based method was more robust and reliable, especially when localizing a leak. The team’s studies show that an alternative method can be applied for shaft-driven piston pumps having either an integrated valve block or split blocks with a high-vibration transfer. Leak localization for this kind of pump is mainly based on the phase of the pulsating vibration level. The team can use it to localize one dominating leaky valve at a time.